Invertebrate Design by Owen Borville November 19, 2024 Biology 28
An endoskeleton is an internal skeleton of an organism covered by an epidermis in some invertebrates and vertebrates.
Porifera is called the simplest of all invertebrates is a phylum that contains the marine sponges. Porifera do not have true tissue, but have specialized cells that perform particular functions like tissues. The external pinacoderm of sponges is similar to our epidermis but not identical.
Sponge larvae are flagellated and swim, but adults are non-motile and are attached to an object on the seafloor (substrate). Water travels through the body of the sponge, providing the sponge with food and other body functions allowing waste and gas to move through out the body.
Sponge morphology is an irregular cylinder shape with a large central cavity called the spongocoel in which water enters through many pores called ostia. The water entering the spongocoel is released through a large opening called the osculum.
Sponges have tissue-like material composed of specialized cells that perform distinct functions. Epithelial-like cells called pinacocytes make up the outermost part of the body, called a pinacoderm, for protection.
Ostia are scattered on the pinacoderm that allow water to enter the body. The ostia pores are formed by porocytes, tube-shaped cells that regulate the flow of water. In other sponges, ostia are formed by folds in the body wall of the sponge.
The mesohyl is the jelly-like substance between the outer layer and the feeding chambers of the sponge, which contains collagenous fibers. Inside the mesohyl are various types of cells like amoebocytes, which function like stem cells in sponges, and sclerocytes, which produce skeletal materials. The mesohyl is like an endoskeleton and maintains the tubular morphology of sponges.
The feeding chambers inside the sponge are lined by choanocytes, or collar cells, that generate a directed water current through the sponge and trap and ingest microscopic food particles by phagocytosis. The body of the choanocyte is embedded in mesohyl and includes all of the essential organelles. The open space inside the feeding chamber has a mesh-like collar composed of microvilli with a single flagellum in the center of the column. The flagella helps draw water into the sponge. Food particles are ingested as they are trapped by the choanocytes. Choanocytes also can differentiate into sperm cells for sexual reproduction by being dislodged from the mesohyl and leave the sponge.
Amoebocytes move through the mesohyl like an amoeba and deliver nutrients from choanocytes to other cells throughout the sponge, and also create eggs for sexual reproduction, while sperm cells are released into the water. The amoebocytes can have other functions, like differentiating into collenocytes and lophocytes that produce collagen-like proteins that support the mesophyll. Amoebocytes can also produce sclerocytes, which produce spicules that are skeletal spikes of silica or carbon-carbonate. Amoebocytes can also produce spongocytes that produce the protein spongin in the majority of sponges.
Sponges are classified by the presence and composition of spicules. Some sponges produce calcium carbonate spicules, some sponges produce siliceous or glassy spicules. Some sponges may or may not have spicules but have spongin. Coralline sponges have multi-layered calcium carbonate skeletons.
Digestion in sponges occurs as their food is trapped as water passes through the ostia and out through the osculum. All digestion in sponges is intracellular. Larger food particles can be phagocytized at the sponge surface by pinacocytes. Other body functions are performed by diffusion between cells that line the openings within the sponge and water passing through. Some sponges have endosymboitic relationships with green algae or cyanobacteria. There is no nervous system in sponges other than intercellular communication that regulate body activities.
Sponges reproduce sexually and asexually. Asexual reproduction is performed by fragmentation or budding. Gemmules are environmentally resistant structures produced by adult sponges where an inner layer of archeocytes or amoebocytes are surrounded by a pneumatic cellular layer that can be reinforced with spicules and help the sponge survive extreme environmental conditions. Sexual reproduction in sponges occurs with gamete generation. Oocytes arise by the differentiation of amoebocytes and are retained in the spongocoel, where the spermatozoa result from the choanocytes and are ejected by the osculum. Sponges are monoecious or hermaphroditic and produce both gametes including sperm and egg. Early larva development occurs within the sponge before it is released.
Sponges generally are immotile and cannot move, however, sometimes sponges move by rearranging their cells along the substrate and by creeping, as observed in the laboratory.
Phylum Cnidaria
Cnidaria is a phylum group of mostly marine animals that have radial or biradial symmetry and form from two embryonic layers, ectoderm and endoderm (diploblastic). The most important cell type for Cnidaria is the cnidocyte, or stinging cell, which is located around the mouth and tentacles and helps capture prey or repel predators. Cnidocytes have large stinging organelles called nematocysts that usually contain barbs at the base of a long coiled thread. A hairlike projection called a cnidocil is sensitive to stimulation and if t0uched, hollow threads turn outward with great acceleration, 40,000 times the acceleration of gravity. The threads entangle the prey or penetrate the flesh, releasing toxins and stopping the prey.
Two different body forms are found in Cnidaria: the polyp tulip-like stalk or the medusa-like bell form. The polyp form is found in the Hydra genus, while the medusa bell form is found in the sea jellyfish. Polyps are sessile as adults with mouth and an anus, while the medusa bell is motile with a mouth and tentacles. Some Cnidarians are dimorphic, and have both forms in their life cycle, such as with Obelia.
All Cnidarians are diploblastic and have two epithelial layers. The outer ectoderm is the epidermis, while the inner layer, the endoderm, is the gastrodermis and lines the digestive cavity. A non-living, jelly-like mesoglea is between the two epithelial layers. The nervous system in Cnidaria contains nerve cells organized in a network across the body, or nerve net that has groups of cells that form nerve plexi. The gastrovascular cavity has a mouth and anus and cells undergo diffusion in the gastrovascular cavity, exchanging oxygen and carbon dioxide. Phylum Cnidaria is divided into the Anthozoa, which incudes sessile polyp forms like the corals, sea fans, sea whips, and sea anemones. The Medusozoa includes the hydrozoa and jellies, both polyp and medusa forms.
Class Anthozoa is known as the sea flowers and includes sea anemones, sea pens, and corals. These are cylindrical and sessile. The mouth of the sea anemone is surrounded by tentacles that have cnidocytes. Anemones are bilateral symmetrical with a slit-like mouth opening and flat pharynx where grooves on each side direct water into the body. The pharynx is the muscular part of the digestive system that helps ingestion. The gastrovascular cavity is divided into several chambers by longitudinal septa called mesenteries. Sea anemones feed on small fish and shrimp, and some develop a mutualistic relationship with hermit crabs. Clownfish also have a mutual relationship with anemones. Anthozoans can reproduce asexually and sexually.
Class Scyphozoa includes the marine jellies that undergo both the polyp and medusa life stages. In the sea jelly, the mouth is located on the underside of the animal, surrounded by hollow tentacles with nematocysts. Scyphozoans are free swimming carnivores. Its mouth leads to the gastrovascular cavity, which can be divided into sacs. Some species have radial canals for digestion. The gastrovascular cells help with ingestion and body support, just like with Anthozoans. Scyphozoa nerve cells are organized in a nerve net over the entire body, with a central nerve ring around the edge of the bell. There is also a ring of muscles that line the dome of the body that allow movement. Scyphozoans reproduce both sexually and asexually. The polyp stage reproduces asexually by budding, while the medusa stage reproduces sexually.
Class Cubozoa are jellies that have a box or cube-shaped medusa bell and are known as box jellyfish. Cubozoans are similar to Scyphozoans except that the Cubozoan tentacles are arranged differently. Muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. Cubozoans are unique in that they have image-forming eyes, including a corona, lens, and retina. Cubozoans reproduce sexually and fertilization occurs inside the female. Planula larvae develop inside the female and are released, then later develop into a polyps and later into a single medusa.
Class Hydrozoa are mostly marine animals with both the polyploid and medusa form in their life cycles. The medusa form has a muscular veil or velum. The polyp form commonly shows cylindrical morphology with a central gastrovascular cavity lined with the gastrodermis. The gastrodermis and epidermis have a layer of mesoglea in between. The mouth is surrounded by tentacles at the oral end of the animal. Siphonophores are a free-floating species that contain both polyploid and medusa forms and are specialized for feeding, defense, and reproduction. The Portuguese Man of War (Physalia physalis) floats by balancing gas exchange in its body, and appears to be a single organism. However, there are several different organisms that make up the man of war, making a colony that resembles a single animal. The man of war has male and female colonies that release gametes into the water. Zygotes develop, then buds asexually to form a new colony.
Superphylum Lophotrochozoa contains animals that are triploblastic with three germ layers and have an embryonic mesoderm between the ectoderm and endoderm. Lophotrochozoa are bilaterally simmetrical and have nervous and sensory organs in the head of the organism. Lophotrochozoa are also protostomes in which the blastopore or the point of invagination of the ectoderm becomes the mouth opening into the alimentary canal, which is also called "first mouth." The coelom is a cavity that separates the ectoderm from the endoderm. Sometimes the forming body cavity splits the mesoderm, called a schizocoelom. Most of Lophotrochozoa have either a lophophore feeding apparatus or a trochopore larvae.
Phylum Platyhelminthes includes the flatworms that include free-living and parsitic forms. The Platyhelminthes include Catenulida or chain worms that reproduce asexually by budding, but the offspring do not fully detach and form a chain. Rhabdites are rodlike structures discharged in the mucus produces by some free-living flatworms that function for defense and traction. Flatworms have three embryonic tissue layers that produce epidermal tissues and the lining of the digestive system along with other internal tissues. The epidermal tissue is a single layer of cells or a layer of fused cells called syncytium that covers two layers of muscle. Mesodermal tissues contain cells with collagen and support secretory cells that produce mucus. Flatworms are acoelomates (no coelom) and the mesoderm layer forms a solid mass. Flatworms are predators or scavengers, including parasitic forms. Flatworms have a branching gastrovascular cavity where the mouth is also used for waste disposal or anus. Tapeworms don't have a digestive system, but absorb digested food from their host. Flatworms have an excretory system with a network of tubules attached to flame cells, whose cilia beat in order to remove waste fluids out of the body. Flatworms also have a pair of lateral nerve chords that run across the length of the body with transverse connections between them. Two large cerebral ganglia are concentrations of nerve cells at the head of the worm that function as sensory organs (photo and chemical). Diffusion allows for gas and nutrient exchange, and there is no circulatory or respiratory system. Most flatworms have both reproductive organs and fertilization is internal. Asexual reproduction also occurs within some flatworms.
Flatworms have four classes or divisions: Turbellaria, Monogenea, Trematoda, and Cestoda. Free-living flatworms can live in marine, freshwater, or moist terrestrial environments. The ventral epidermis of flatworms is ciliated, which allows for locomotion. Some flatworms are capable of regeneration of body parts, including the head or tail. Turbellaria is a free living flatworm, while Monogeneans are fish parasites. Trematodes (flukes) are internal parasites of mollusks and other animals, including humans. Cestodes, or tapeworms, are also internal parasites of vertebrate animals.
Phylum Rotifera (commonly called wheel-bearers or rotifers) contains microscopic aquatic animals that have a corona, a pair of ciliated-feeding structures that appear to rotate. The rotifer body is divided into three sections: the head (and corona), trunk (internal organs), and foot. A cuticle covers the body surface. Muscles are associated with locomotion and digestion. Rotifers are free-swimming or planktonic (drifting) organisms. Sticky secreted material can help rotifers attach to surfaces. In the head, there are eyespots and nerves that spread throughout the body. Rotifers are found in freshwater and saltwater worldwide and eat dead material, algae, and other organisms, and therefore are important in the ecological food web. Food particles enter the mouth and the mastax, a muscular pharynx with teeth and jaws. There are digestive glands, a stomach, and intestines. Wastes are collected in the bladder and released from the anus. Some rotifers are dimorphic where males and females have different forms.
Phylum Nemertea species are known as ribbon worms or proboscis worms, where most are marine and benthic or bottom dwellers. Nemerteans have been found in freshwater and wet terrestrial environments. Most nemerteans are carnivores, scavengers, or form commensal relationships with mollusks. Nemerteans can be one centimeter to several meters in length, have bilateral symmetry, and have contractile properties where they can change their morphology because their bodies are soft, unsegmented tubes. Nemerteans commonly have a rhynchocoel, a long body length central fluid filled cavity that includes a probosics, a tentacle for food capture that can be extended by hydrostatic pressure and muscle action. Nemerteans have a digestive system from the mouth opening to the foregut, to the intestines, to the anus. Gonads or sex organs are located in the intestine. The circulatory system is a closed loop formed by a connected pair of blood vessels. Protonephridia pairs are excretory tubules. The nervous system contains four ganglia at the anterior end around the rhynchocoel. Paired nerve cords extend from the ganglia to the other end. Nemerteans have capability of regeneration of body parts, and asexual reproduction by fragmentation. Most nemerteans are dioecious (separate male and female individuals), but some are hermaphroditic (both sex organs). Gametes combine with gonads in the digestive tract. Fertilization occurs externally. Nemerteans have spiral cleavage.
Phylum Mollusca are soft-bodied marine animals that can live in freshwater and terrestrial environments and includes chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses. The morphology of phylum Mollusca varies greatly but there are some common characteristics. Most mollusks have a muscular foot for locomotion and most internal organs are located in the visceral mass. Above the visceral mass is a fold of tissue called the mantle, and inside the cavity formed by the mantle are respiratory structures called gills, that usually fold over the visceral mass. The mouths of most mollusks contain a tongue like feeding organ called the radula. The mantle secretes a calcium-carbonate hardened shell in most mollusks. The muscular foot is the most ventral organ and the mantle is the organ that folds over the visceral mass. The visceral mass contains digestive, nervous, excretory, reproductive, and respiratory systems. Most mollusks have an organ, the radula, that has teeth made of chitin. The mantle is the dorsal epidermis in mollusks. Most mollusks can secrete a calcareous shell for protection of its soft body. Mollusk fertilization occurs externally in aquatic varieties, but not terrestrial ones. In aquatic mollusks, the zygote hatches and produces a trochophore larva with cilia. Some larvae undergo a second larvae stage (veliger larvae) before becoming an adult.
Phylum Mollusca is divided into Caudofoveata, Solenogastres, Monoplacophora, Polyplacophora, Gastropoda, Cephalopoda, Bivalvia, and Scaphopoda.
Caudofoveata and Solengastres contain shell-less, worm-like animals in benthic marine habitats with calcareous spicules in their epidermis offering protection and the mantle cavity is smaller and there are no eyes, tentacles, or excretory organs. Caudofoveata has radula, but Solengastres do not have radula or gills. Caudofoveata has no foot.
Monoplacophora have one cap-like shell that covers the body, radula, digestive system, pairs of excretory organs, and a pair of gonad sex organs along with several gills.
Polyplacophora have many plates or chitons covering their body, a foot with suction capability, and a mantle that extends beyond the outer shell containing calcareous spines for protection. Multiple gills offer respiration in the mantle cavity. Blood from gills is collected in the heart and sent to the rest of the body in the hemocoel, an open circulatory system where the blood is contained in connected chambers around the organs rather than in individual blood vessels. The radula have teeth composed of magnetite, harder than any teeth in the biological world. The nervous system has ganglia at the anterior end with sensory structures, including photosensors that extend from the mantle to the channels of the upper shell called esthetes. Other sensory structures help find food. A pair of nephridia is used for excretion of nitrogenous wastes.
Bivalvia have two valves and includes the clams, oysters, muscles, scallops, geoducks, and shipworms. Bivalves range from microscopic to one meter in length and are found in marine and freshwater habitats. The two valves of the organism enclose the body, and the valves are fused together with hinge ligaments and teeth that keep the two halves aligned. The two shells contain an outer organic layer, a middle-prismatic layer, and a smooth nacreous layer, are joined at the umbo. Muscles of the bivalve help open and close the shell. The body of the bivalve is flat laterally, and the foot is wedge-shaped. Bivalves are filter-feeders and do not have radula. The mantle cavity is fused along the edges with openings for the foot and openings for the circulation of water in and out by siphons. During water intake, siphons capture food particles by the ctenidia gills and carried by cilia to the mouth. Excretion and osmoregulation are performed by a pair of nephridia. Eyespots and other sensory structures are located on the edge of the mantle, particularly noticeable in scallops. Three pairs of connected ganglia regulate activity of different body structures. The mantle functions to secrete the shell. Some bivalves can produce calcareous nacre material that can be used to produce pearls.
Gastropoda makes up more than half of molluscan species and are known for their "stomach foot," including snails, slugs, conchs, cowries, limpets, and whelks. These are found in aquatic marine and freshwater habitats, and terrestrial habitats. Some gastropods have shells and some do not. Gastropod bodies are asymmetrical and commonly have a coiled shell, which can be planospiral like garden snails or conispiral like conches. Some shell spirals are covered by the mantle. Gastropod embryonic development involves torsion, a unique process that occurs when the visceral mass, mantle, and shell of a gastropod rotate 180° in a counterclockwise direction relative to the head and foot. The torsion process produces the asymmetric shape of the developed gastropod. Gastropods have a foot used for crawling, a head with tentacles, eyes, and radula (tongue) used to obtain food particles. In aquatic gastropods, the mantle cavity encloses the gills (ctenidia), however, in land gastropods, the mantle works as the respiratory system and functions like a lung. Nephridia (kidneys) are located in the mantle cavity.
Cephalopoda are known as the head-foot animals and include octopuses, squids, cuttlefish, and nautiluses. Some cephalopods have shells and some have no shell. The nautilus has a multi-chambered spiral shell that fill with gas or water to regulate buoyancy. A siphuncle tube passes through the chambers and regulates the amount of water and gases in each chamber. Ammonites and other nautiloid shells are common in the fossil record. Squids and cuttlefish have smaller shells and that exist internally in the body and are known as squid pen and cuttlefish bone, respectively. Cephalopods can rapidly change their body color, particularly in squid and octopus, and is used for camouflage or mating signals. Pigment cells (chromatophores) are pigment cells in the skin can expand or contract to produce different color patterns. Underneath the chromatophores are iridophores and leucophores that can produce a greater variety of colors. Cephalopods are carnivores that have jaws and radula. Cephalopod nervous systems have image-forming eyes. Cephalopods have a closed circulatory system with blood vessels. The foot is lobed and divided into arms and tentacles. Suckers with rings are found on the arms and tentacles of octopuses and squid. Siphon tubes are used for locomotion by expulsion of water as propulsion. Gills (ctenidia) are located in the mantle cavity walls and large blood vessels each have their own heart. Nephridia organs are present in the mantle cavity for water balance and waste disposal. Cephalopods including octopuses and squid produce sepia or dark ink which contains melanin between the gills and can be released into the water to produce a smoke screen to hide from predators. Cephalopods are dioecious as member species mate and the female lays eggs that hatch and develop without a larva stage.
Scaphopoda are known as boat feet, tusk shells, or tooth shells that are usually buried in the sand with only the anterior opening exposed. These have a single conical shell open on both ends. There is a head, a mouth with radula, and surrounded by tentacles with bulbs that can catch prey. Scaphopods also have a foot, but there is no ctenidia (gill), but the mantle cavity serves as a tube that extends to both ends and allows for respiration.
Phylum Annelida includes the segmented worms found in marine, freshwater, and terrestrial environments. Annelids require water or humidity for survival in terrestrial environments. Annelids are called segmented worms because their bodies have metamerism or true segmentation. Marine annelids, earthworms, and leaches are annelids and some are parasitic or symbiotic. Annelids have bilateral symmetry with ring-like segmentation. The metameric segmentation where several internal and external morphological features are repeated in each segment and the organism can become bigger by adding new segments and improving their movement. The annelid body contains a head, body, and pygidium (tail). As the annelid develops, segments behind the head appear sequentially from growth from the tail, or teloblastic growth. In the Oligochaetes, the clitellum is a reproductive structure that generates mucus to aid sperm transfer and produces a cocoon, where fertilization occurs. The epidermis is protected by a cuticle and muscles are located inside the epidermis. Setae are chitinous bristles set or anchored in the epidermis and connected to muscles. In the polychaetes, setae are born on paired appendages called parapodia. Most annelids have a digestive system and feeding mechanisms vary, such as with filter-feeders, tentacles, jaws, or pharynx. Earthworms collect food as they burrow through the soil. Earthworm digestive tracts include a mouth, muscular pharynx, esophagus, crop, and muscular gizzard. The gizzard leads to the intestines, which ends in the anal opening in the terminal segment. Most annelids have a closed circulatory system of blood vessels that run parallel to the digestive tract and capillaries that connect to individual tissues. The vessels are connected by transverse loops in every segment. Some polychaetes and leaches have open circulatory systems where blood vessels open into a hemocoel. Annelid respiration occurs as gas exchange across the body surface. In polychaetes, vascular parapodia serve as respiratory structures. Excretion occurs with a metanephridia tubule waste disposal system in every segment. A ring of fused ganglia around the pharynx serve as the nervous system. A ventral nerve cord has nodes in each segment. Annelids can be monoecious with permanent gonad sex organs as in earthworms or leaches or dioecious with temporary gonads as in polychaetes, but cross-fertilization is preferred. In polychaetes, fertilization is external, while in oligochaetes, fertilization is internal. Polychaetes can regenerate and some can reproduce asexually. Phylum Annelida classification contains the polychaetes (many bristles), and the oligochaetes (few bristles; earthworms, leaches).
Superphylum Ecdysozoa includes the Nematodes, Tardigrades, and Arthropods. The most distinguishing feature of this superphylum is the cuticle, or exoskeleton that protects these animals from dehydration, predators, and other environmental factors. All members of superphylum Ecdysozoa also shed their old exoskeleton after a new one grows underneath it by secretion.
Phylum Nematoda animals are triploblastic (endoderm, mesoderm, ectoderm), bilaterally symmetrical, are pseudocoelomates, and show tubular morphology and a circular cross section. Some nematodes are parasitic. Nematodes live in all habitats and are abundant through out the world. The nematode body is cylindrical and has a digestive system with a mouth and anus. The mouth connects to a muscular pharynx and intestine, rectum, and anal opening at the other end. The epidermis can be a single layer of cells or a syncytium, a multinucleated tissue that is formed by the fusion of many single cells. The cuticle of nematodes is rich in collagen and a polymer called chitin, which forms a protective armor outside the epidermis. The cuticle extends into both ends of the digestive tract, the pharynx, and rectum. In the head, the anterior mouth opening is composed of three (or six) lips and teeth derived from the cuticle in some species. Some nematodes also have rings, head shields, or warts, but the rings do not represent segmentation. Muscle attachment in nematodes is unique in that they have a longitudinal layer only and are directly attached to the dorsal and ventral nerve cords, which creates a strong muscular contraction that results in a whiplike body movement. In nematodes, wastes are released directly across the body wall or osmoregulation and salt balance are performed by excretory cells (renette cells) or glands that can be connected to canals that release wastes through pores. Nematodes have four longitudinal nerve cords that run the length of the body in dorsal, ventral, and lateral positions and these cords fuse at the anterior end, to form a pharyngeal nerve ring around the pharynx, which acts as the head ganglion or brain of the nematode roundworm. A similar fusion occurs at the other end of the body. Nematode reproduction methods can vary, from monoecious, where one organism has both sex organs, to dioecious, where each organism has either one sex organ or the other, to parthenogenic, with no male partner. Fertilization is internal, and the embryo develops soon after fertilization. Parasitic nematodes include guinea worms, hookworms, Trichina worms, Ascaris, Dirofilaria, and Wuchereria. These are a threat to other animals and humans from contaminated water.
Phylum Tardigrada are known as slow steppers or water bears and are small animals living in marine, freshwater, and wet terrestrial environments. Tardigrades have a plump body, short legs, and claws. A chitinous cuticle covers the body surface and can be divided into plates. Tardigrades are known for their ability of cryptobiosis, a state of resistance to many environmental difficulties and extreme conditions, including temperature, pressure, dryness, and radiation. Tardigrades can pause their body functions for years and survive with low levels of water because of specialized proteins that replace water in their cells and protect their cellular DNA structure from damage. Tardigrades have cylindrical bodies and four pairs of legs with claws. The first three pairs of legs are used for walking, while the rear pair of legs are used for attachment to a surface. The mouth is connected to a muscular pharynx and salivary glands as they feed on plants, algae, or small animals. Muscle cells extend from the epidermis to the legs and help in movement. The hemocoel is the major body cavity where tubules remove metabolic waste and transports them through the body. The dorsal brain is connected to the ventral nerve cord with segmented ganglia in the appendages. Sensory structures include eyespots, cilia, or bristles. Most tardigrades are dioecious and each individual tardigrade has a certain gonad, male or female. Fertilization is external and development produces a fixed number of cells that enlarge but do not multiply.
Phylum Arthropoda or arthropods, are coelomates (internal fluid chamber) with a chitinous exoskeleton and jointed appendages. Arthropods go through the process of molting and ecdysis (shedding of exoskeleton). Arthropods are protostomic in their life cycle. Arthropod means jointed feet. Arthropod characteristic morphology includes a segmented body both structurally and functionally, jointed appendages, and waterproof exoskeleton made of chitin. Arthropods live in terrestrial, aquatic, and aerial habitats. Arthropoda includes trilobites, horseshoe crabs, spiders, scorpions, ticks, mites, daddy long legs or harvestmen, millipedes, centipedes, crustaceans, crabs, lobsters, crayfish, and insects of six legs. Trilobites are extinct and can be seen in the fossil record. Arthropod characteristics include jointed appendages, body segmentation, and chitin exoskeleton. Functional body regions (tagmata) include the head, thorax, abdomen, cephalothorax and abdomen, or head and trunk. The arthropod central cavity is the hemocoel blood cavity and fluids are moved by contraction of regions by the tubular dorsal blood vessel. Some arthropods use green glands or Malpighian tubules for waste excretion. The nervous system is distributed along the segments and a ventral nerve cord. Arthropod respiratory systems vary. Tubes or trachae branch through the body to tracheoles that perform gas exchange. Trachea open to the surface of the cuticle via apertures called spiracles. Aquatic crustaceans use gills, terrestrial chelicerates have book lungs, and aquatic chelicerates use book gills. Book lung tissue looks like the pages of a book. The gills of crustaceans exchange gases with the surrounding water.
The cuticle is the hard covering of the arthropod, and it has two layers: the epicuticle is a thin and water resistant outer layer with no chitin, while the layer beneath is chitinous procuticle, with an upper and lower layer. The exoskeleton is made of chitin bound with a protein. The procuticle is flexible, lightweight, and protects against dry environment. The arthropod grows by shedding its exoskeleton by molting and ecdysis (stripping).
Subphylum Chelicerata includes horseshoe crabs, sea spiders, spiders, mites, ticks, scorpions, and harvestmen. Most of these are terrestrial, but some are aquatic freshwater and marine. The body of chelicerates are divided into two tagmata body parts: prosoma and opsithosoma, which are similar to the cephalothorax and abdomen. A distinct head is usually unnoticeable. The chelicerae is the first pair of appendages with claw-like mouthparts. The second pair of appendages is called the pedipalps and can be used for sensing or food manipulation. Some species have an extra pair of appendages between the chelicerae and pedipalps called ovigers, used for grooming and carrying eggs. The nervous system in chelicerates contains a brain and two ventral nerve cords. Chelicerates are dioecious with separate sexes for reproduction and use external and internal fertilization.
Subphylum Myriapoda includes the arthropods (centipedes and millipedes) with numerous legs, but usually between 10 to 750. Myriapoda are found in humid terrestrial environments such as moist soils and decaying biomaterial such as leaves. Centipedes have one pair of legs per segment connected laterally, mouthparts, and somewhat flattened. Centipedes are predators and have poison claws to capture prey. Millipedes have two pairs of legs per diplosegment attached underneath, rounder in cross-section than centipedes, and are herbivores.
Subphylum Crustacea are dominant in freshwater and marine environments. Krill, shrimp, lobster, crabs, and crayfish are types of crustaceans. Terrestrial crustaceans include pill bugs, roly-polies, potato bugs, and isopods. Crustaceans have two pairs of antennae, mandibles as mouthparts, two branched appendages so that their legs are formed in two parts: endopods and exopods. Insects have one branched or unibranched appendages. In most crustaceans, the head and thorax are fused together to form the cephalothorax, which is covered by a plate called a carapace. The body plan has two tagmata: cephalothorax and abdomen. A chitinous exoskeleton is shed by molting. Crustaceans have an open circulatory system where blood is pumped into the hemocoel by the heart. Most crustaceans are dioecious, but barnicles may be hermaphrodites. Serial hermaphroditism is where the gonad can switch from producing sperm to ova in some species. Larval stages are seen in early development aquatic crustaceans. Crustaceans have a brain formed by the fusion of the first three segmental ganglia and two compound eyes. A ventral nerve cord connects additional ganglia. Most crustaceans are carnivorous, but some are herbivorous or even parasitic.
Subphylum Hexapoda is the largest class of arthropods and includes the insects. The common feature is six legs or three pairs of legs. Hexapod bodies are divided into three tagmata: head, thorax, and abdomen. Mouthparts are on the head and the thorax has three pairs of jointed appendages, along with wings. Pterygotes are the winged insects. THe head has an upper lip and lower lip that serve as mouthparts along with sensory antennae, compound eyes, ocelli (simple eyes), and sensory hairs. The abdomen has 11 segments along with reproductive parts. The evolution of wings is difficult to explain for established scientists. Many insects develop by metamorphosis from larvae to adult, such as with ants, beetles, flies, and butterflies. Some insects work in groups or colonies and use pheromones, external chemical signals, to communicate with each other.
Phylum Echinodermata and Chordata, which includes humans, belongs to the superphylum Deuterostomia. Protostomes and deuterostomes are named based on which opening of the archenteron, or gut tube, develops first. In deuterostomes, the mouth develops as a secondary structure opposite the blastopore, which becomes the anus. In protostomes, or mouth first, the first embryonic opening becomes the mouth, and the second opening becomes the anus.
More differences between protostomes and deuterostomes include: type of embryonic cell division or cleavage, mode of formation of the coelom of the embryo: Protostomes have spiral mosaic cleavage while deuterostomes have radial cleavage. In deuterostomes, the endodermal lining of the archenteron usually forms buds called coelomic pouches that expand and ultimately destroy the embryonic blastocoel, the cavity within the blastula and early gastrula to become the embryonic mesoderm, the third germ layer. This happens when the mesodermal pouches become separated from the invaginating endodermal layer forming the archenteron, then expand and fuse to form the coelomic cavity. The resulting coelom is called an enterocoelom. The archenteron develops into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the blastopore of the gastrula. The blastopore forms the anus of the alimentary system in the juvenile and adult forms. Cleavage in most deuterostomes are also indeterminant, meaning that the developmental fates of early embryonic cells are not decided at that point of embryonic development.
The deuterostomes consist of two major clades: the Cordata and the Ambulacraria. The Chordata includes the vertebrates and two invertebrate subphyla (urochordates and cephalochordates). The ambulacraria include the echinoderms and the hemichordates.
Most invertebrates do not have a bony vertebral endoskeleton, or bony cranium. However, Echinodermata produce tiny skeletal bones called ossicles that create an endoskeleton, or internal skeleton covered by an epidermis.
Echinodermata is named for their prickly skin, marine, bottom-dwelling habitats including sea stars, sea cucumbers, sea urchins, sand dollars, and brittle stars. Echinoderms have pentaradial symmetry with arms around a central axis. They have an endoskeleton made of calcareous ossicles, or small bony plates, covered by the epidermis. In some species, the spines are surrounded by tiny stalked claws called pedicellaria, which help keep the animal's surface clean of debris, protect papulae used in respiration, and sometimes aid in food capture.
The endoskeleton is produced by dermal cells, which also produce several kinds of pigments. Sea stars have finger like projections (papilae) of dermal tissue extend through the endoskeleton and function like gills. Some cells are toxic. Each arm contains different structures: digestive glands, gonads, and tube feet. Every arm bears two rows of tube feet on the oral side along an external ambulacral groove. The tube feet assist in locomotion, feeding, and chemical sensations, as well as serve to attach some species to the substratum.
Echinoderms have a unique ambulacral water vascular system, derived from the coelom body cavity. The water vascular system contains a central ring canal and radial canals that extend along each arm. Each radial canal is connected to a double row of tube feet, which project through holes in the endoskeleton, and function as tactile and ambulatory structures. The tube feet can extend and retract based on the volume of water present in the system of that arm, allowing the animal to move and also allowing it to capture prey with their suckerlike action. Individual tube feet are called ampullae. Sea water enters the system through the aboral madreporite and passes to the ring canal through a short stone canal. Water circulating through these structures facilitates gaseous exchange and provides a hydrostatic source for locomotion and prey manipulation. A hemal system, consisting of oral, gastric, and aboral rings, as well as other vessels running roughly parallel to the water vascular system, circulates nutrients. The water vascular and the hemal system transport nutrients and gases along with the visceral body cavity.
The nervous system of echinoderms has a circumoral nerve ring at the center and five radial nerves extending outward along the arms. Also, several networks of nerves are located in different parts of the body. The mouth opens to an esophagus to a stomach. Digestive glands (pyloric caceca) in each arm, running dorsally along the arms and the reproductive glands below them. Then digested food is transferred to the anus. Podocytes are cells specialized for ultrafiltration of bodily fluids are located near the center of the echinoderm disc. These podocytes are connected by an internal system of canals to the madreporite, where water enters the stone canal. The adult echinoderm usually has a large, fluid filled coelom. Cilia aid in circulating fluid within the body cavity, and lead to the fluid filled papulae, where the exchange of oxygen and carbon dioxide take place, as well as the secretion of nitrogenous waste such as ammonia, by diffusion. Echinoderms are dioecious, but males and females are indistinguishable aside from their gametes, which are released into water and fertilization is external. Early larval stages of echinoderms have bilateral symmetry and the radially symmetric adult forms a cluster of cells in the larvae. Echinoderms may also reproduce by asexual fragmentation.
The Phylum of Echinoderms are divided into five classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lillies and feather stars), and Holothuroidea (sea cucumbers). Sea stars have thick arms that extend from a central disk and various organs branch into the arms. Each arm has eyes and sensory tentacles. Sea stars use their rows of tube feet for attaching to surfaces and grasping prey. Sea stars are mostly carniverious and have two stomachs. They use their arms to open bivalve molluscs. Brittle stars (snake-tails) have long, thin, flexible arms that extend from their central disk and are used for locomotion and grasping prey. Sea urchins and sand dollars do not have arms, but are hemispherical or flattened with five rows of tube feet that extend through five rows of pores in a continuous internal shell called a test. Their tube feet help keep their body surface clean as they feed on algae. Sea lilies and feather stars are Crinoids. Sea lilies are sessile and attached to a stalk, but feather stars can move with their leglike cirri. Sea lilies and feather stars are both suspension feeders and their arms are used to catch food. Sea cucumbers are the only echinoderms that are bilaterally symmetrical as adults, as they lie horizontally. Tube feet are little or none, and there is a single gonad. Respiratory trees are gill-like structures that branch from the gut and muscles pump water in and out. Tentacles cluster around the mouth, and eat detritus or suspension organisms.
Phylum Chordata has five key features: a notochord, a dorsal hollow nerve cord, pharyngeal slits, a post-anal tail, and an endostyle/thyroid gland that secretes iodinated hormones. Chordata contains vertebrate classes and two invertebrate classes: Urochordata and Cephalochordata.
An endoskeleton is an internal skeleton of an organism covered by an epidermis in some invertebrates and vertebrates.
Porifera is called the simplest of all invertebrates is a phylum that contains the marine sponges. Porifera do not have true tissue, but have specialized cells that perform particular functions like tissues. The external pinacoderm of sponges is similar to our epidermis but not identical.
Sponge larvae are flagellated and swim, but adults are non-motile and are attached to an object on the seafloor (substrate). Water travels through the body of the sponge, providing the sponge with food and other body functions allowing waste and gas to move through out the body.
Sponge morphology is an irregular cylinder shape with a large central cavity called the spongocoel in which water enters through many pores called ostia. The water entering the spongocoel is released through a large opening called the osculum.
Sponges have tissue-like material composed of specialized cells that perform distinct functions. Epithelial-like cells called pinacocytes make up the outermost part of the body, called a pinacoderm, for protection.
Ostia are scattered on the pinacoderm that allow water to enter the body. The ostia pores are formed by porocytes, tube-shaped cells that regulate the flow of water. In other sponges, ostia are formed by folds in the body wall of the sponge.
The mesohyl is the jelly-like substance between the outer layer and the feeding chambers of the sponge, which contains collagenous fibers. Inside the mesohyl are various types of cells like amoebocytes, which function like stem cells in sponges, and sclerocytes, which produce skeletal materials. The mesohyl is like an endoskeleton and maintains the tubular morphology of sponges.
The feeding chambers inside the sponge are lined by choanocytes, or collar cells, that generate a directed water current through the sponge and trap and ingest microscopic food particles by phagocytosis. The body of the choanocyte is embedded in mesohyl and includes all of the essential organelles. The open space inside the feeding chamber has a mesh-like collar composed of microvilli with a single flagellum in the center of the column. The flagella helps draw water into the sponge. Food particles are ingested as they are trapped by the choanocytes. Choanocytes also can differentiate into sperm cells for sexual reproduction by being dislodged from the mesohyl and leave the sponge.
Amoebocytes move through the mesohyl like an amoeba and deliver nutrients from choanocytes to other cells throughout the sponge, and also create eggs for sexual reproduction, while sperm cells are released into the water. The amoebocytes can have other functions, like differentiating into collenocytes and lophocytes that produce collagen-like proteins that support the mesophyll. Amoebocytes can also produce sclerocytes, which produce spicules that are skeletal spikes of silica or carbon-carbonate. Amoebocytes can also produce spongocytes that produce the protein spongin in the majority of sponges.
Sponges are classified by the presence and composition of spicules. Some sponges produce calcium carbonate spicules, some sponges produce siliceous or glassy spicules. Some sponges may or may not have spicules but have spongin. Coralline sponges have multi-layered calcium carbonate skeletons.
Digestion in sponges occurs as their food is trapped as water passes through the ostia and out through the osculum. All digestion in sponges is intracellular. Larger food particles can be phagocytized at the sponge surface by pinacocytes. Other body functions are performed by diffusion between cells that line the openings within the sponge and water passing through. Some sponges have endosymboitic relationships with green algae or cyanobacteria. There is no nervous system in sponges other than intercellular communication that regulate body activities.
Sponges reproduce sexually and asexually. Asexual reproduction is performed by fragmentation or budding. Gemmules are environmentally resistant structures produced by adult sponges where an inner layer of archeocytes or amoebocytes are surrounded by a pneumatic cellular layer that can be reinforced with spicules and help the sponge survive extreme environmental conditions. Sexual reproduction in sponges occurs with gamete generation. Oocytes arise by the differentiation of amoebocytes and are retained in the spongocoel, where the spermatozoa result from the choanocytes and are ejected by the osculum. Sponges are monoecious or hermaphroditic and produce both gametes including sperm and egg. Early larva development occurs within the sponge before it is released.
Sponges generally are immotile and cannot move, however, sometimes sponges move by rearranging their cells along the substrate and by creeping, as observed in the laboratory.
Phylum Cnidaria
Cnidaria is a phylum group of mostly marine animals that have radial or biradial symmetry and form from two embryonic layers, ectoderm and endoderm (diploblastic). The most important cell type for Cnidaria is the cnidocyte, or stinging cell, which is located around the mouth and tentacles and helps capture prey or repel predators. Cnidocytes have large stinging organelles called nematocysts that usually contain barbs at the base of a long coiled thread. A hairlike projection called a cnidocil is sensitive to stimulation and if t0uched, hollow threads turn outward with great acceleration, 40,000 times the acceleration of gravity. The threads entangle the prey or penetrate the flesh, releasing toxins and stopping the prey.
Two different body forms are found in Cnidaria: the polyp tulip-like stalk or the medusa-like bell form. The polyp form is found in the Hydra genus, while the medusa bell form is found in the sea jellyfish. Polyps are sessile as adults with mouth and an anus, while the medusa bell is motile with a mouth and tentacles. Some Cnidarians are dimorphic, and have both forms in their life cycle, such as with Obelia.
All Cnidarians are diploblastic and have two epithelial layers. The outer ectoderm is the epidermis, while the inner layer, the endoderm, is the gastrodermis and lines the digestive cavity. A non-living, jelly-like mesoglea is between the two epithelial layers. The nervous system in Cnidaria contains nerve cells organized in a network across the body, or nerve net that has groups of cells that form nerve plexi. The gastrovascular cavity has a mouth and anus and cells undergo diffusion in the gastrovascular cavity, exchanging oxygen and carbon dioxide. Phylum Cnidaria is divided into the Anthozoa, which incudes sessile polyp forms like the corals, sea fans, sea whips, and sea anemones. The Medusozoa includes the hydrozoa and jellies, both polyp and medusa forms.
Class Anthozoa is known as the sea flowers and includes sea anemones, sea pens, and corals. These are cylindrical and sessile. The mouth of the sea anemone is surrounded by tentacles that have cnidocytes. Anemones are bilateral symmetrical with a slit-like mouth opening and flat pharynx where grooves on each side direct water into the body. The pharynx is the muscular part of the digestive system that helps ingestion. The gastrovascular cavity is divided into several chambers by longitudinal septa called mesenteries. Sea anemones feed on small fish and shrimp, and some develop a mutualistic relationship with hermit crabs. Clownfish also have a mutual relationship with anemones. Anthozoans can reproduce asexually and sexually.
Class Scyphozoa includes the marine jellies that undergo both the polyp and medusa life stages. In the sea jelly, the mouth is located on the underside of the animal, surrounded by hollow tentacles with nematocysts. Scyphozoans are free swimming carnivores. Its mouth leads to the gastrovascular cavity, which can be divided into sacs. Some species have radial canals for digestion. The gastrovascular cells help with ingestion and body support, just like with Anthozoans. Scyphozoa nerve cells are organized in a nerve net over the entire body, with a central nerve ring around the edge of the bell. There is also a ring of muscles that line the dome of the body that allow movement. Scyphozoans reproduce both sexually and asexually. The polyp stage reproduces asexually by budding, while the medusa stage reproduces sexually.
Class Cubozoa are jellies that have a box or cube-shaped medusa bell and are known as box jellyfish. Cubozoans are similar to Scyphozoans except that the Cubozoan tentacles are arranged differently. Muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. Cubozoans are unique in that they have image-forming eyes, including a corona, lens, and retina. Cubozoans reproduce sexually and fertilization occurs inside the female. Planula larvae develop inside the female and are released, then later develop into a polyps and later into a single medusa.
Class Hydrozoa are mostly marine animals with both the polyploid and medusa form in their life cycles. The medusa form has a muscular veil or velum. The polyp form commonly shows cylindrical morphology with a central gastrovascular cavity lined with the gastrodermis. The gastrodermis and epidermis have a layer of mesoglea in between. The mouth is surrounded by tentacles at the oral end of the animal. Siphonophores are a free-floating species that contain both polyploid and medusa forms and are specialized for feeding, defense, and reproduction. The Portuguese Man of War (Physalia physalis) floats by balancing gas exchange in its body, and appears to be a single organism. However, there are several different organisms that make up the man of war, making a colony that resembles a single animal. The man of war has male and female colonies that release gametes into the water. Zygotes develop, then buds asexually to form a new colony.
Superphylum Lophotrochozoa contains animals that are triploblastic with three germ layers and have an embryonic mesoderm between the ectoderm and endoderm. Lophotrochozoa are bilaterally simmetrical and have nervous and sensory organs in the head of the organism. Lophotrochozoa are also protostomes in which the blastopore or the point of invagination of the ectoderm becomes the mouth opening into the alimentary canal, which is also called "first mouth." The coelom is a cavity that separates the ectoderm from the endoderm. Sometimes the forming body cavity splits the mesoderm, called a schizocoelom. Most of Lophotrochozoa have either a lophophore feeding apparatus or a trochopore larvae.
Phylum Platyhelminthes includes the flatworms that include free-living and parsitic forms. The Platyhelminthes include Catenulida or chain worms that reproduce asexually by budding, but the offspring do not fully detach and form a chain. Rhabdites are rodlike structures discharged in the mucus produces by some free-living flatworms that function for defense and traction. Flatworms have three embryonic tissue layers that produce epidermal tissues and the lining of the digestive system along with other internal tissues. The epidermal tissue is a single layer of cells or a layer of fused cells called syncytium that covers two layers of muscle. Mesodermal tissues contain cells with collagen and support secretory cells that produce mucus. Flatworms are acoelomates (no coelom) and the mesoderm layer forms a solid mass. Flatworms are predators or scavengers, including parasitic forms. Flatworms have a branching gastrovascular cavity where the mouth is also used for waste disposal or anus. Tapeworms don't have a digestive system, but absorb digested food from their host. Flatworms have an excretory system with a network of tubules attached to flame cells, whose cilia beat in order to remove waste fluids out of the body. Flatworms also have a pair of lateral nerve chords that run across the length of the body with transverse connections between them. Two large cerebral ganglia are concentrations of nerve cells at the head of the worm that function as sensory organs (photo and chemical). Diffusion allows for gas and nutrient exchange, and there is no circulatory or respiratory system. Most flatworms have both reproductive organs and fertilization is internal. Asexual reproduction also occurs within some flatworms.
Flatworms have four classes or divisions: Turbellaria, Monogenea, Trematoda, and Cestoda. Free-living flatworms can live in marine, freshwater, or moist terrestrial environments. The ventral epidermis of flatworms is ciliated, which allows for locomotion. Some flatworms are capable of regeneration of body parts, including the head or tail. Turbellaria is a free living flatworm, while Monogeneans are fish parasites. Trematodes (flukes) are internal parasites of mollusks and other animals, including humans. Cestodes, or tapeworms, are also internal parasites of vertebrate animals.
Phylum Rotifera (commonly called wheel-bearers or rotifers) contains microscopic aquatic animals that have a corona, a pair of ciliated-feeding structures that appear to rotate. The rotifer body is divided into three sections: the head (and corona), trunk (internal organs), and foot. A cuticle covers the body surface. Muscles are associated with locomotion and digestion. Rotifers are free-swimming or planktonic (drifting) organisms. Sticky secreted material can help rotifers attach to surfaces. In the head, there are eyespots and nerves that spread throughout the body. Rotifers are found in freshwater and saltwater worldwide and eat dead material, algae, and other organisms, and therefore are important in the ecological food web. Food particles enter the mouth and the mastax, a muscular pharynx with teeth and jaws. There are digestive glands, a stomach, and intestines. Wastes are collected in the bladder and released from the anus. Some rotifers are dimorphic where males and females have different forms.
Phylum Nemertea species are known as ribbon worms or proboscis worms, where most are marine and benthic or bottom dwellers. Nemerteans have been found in freshwater and wet terrestrial environments. Most nemerteans are carnivores, scavengers, or form commensal relationships with mollusks. Nemerteans can be one centimeter to several meters in length, have bilateral symmetry, and have contractile properties where they can change their morphology because their bodies are soft, unsegmented tubes. Nemerteans commonly have a rhynchocoel, a long body length central fluid filled cavity that includes a probosics, a tentacle for food capture that can be extended by hydrostatic pressure and muscle action. Nemerteans have a digestive system from the mouth opening to the foregut, to the intestines, to the anus. Gonads or sex organs are located in the intestine. The circulatory system is a closed loop formed by a connected pair of blood vessels. Protonephridia pairs are excretory tubules. The nervous system contains four ganglia at the anterior end around the rhynchocoel. Paired nerve cords extend from the ganglia to the other end. Nemerteans have capability of regeneration of body parts, and asexual reproduction by fragmentation. Most nemerteans are dioecious (separate male and female individuals), but some are hermaphroditic (both sex organs). Gametes combine with gonads in the digestive tract. Fertilization occurs externally. Nemerteans have spiral cleavage.
Phylum Mollusca are soft-bodied marine animals that can live in freshwater and terrestrial environments and includes chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses. The morphology of phylum Mollusca varies greatly but there are some common characteristics. Most mollusks have a muscular foot for locomotion and most internal organs are located in the visceral mass. Above the visceral mass is a fold of tissue called the mantle, and inside the cavity formed by the mantle are respiratory structures called gills, that usually fold over the visceral mass. The mouths of most mollusks contain a tongue like feeding organ called the radula. The mantle secretes a calcium-carbonate hardened shell in most mollusks. The muscular foot is the most ventral organ and the mantle is the organ that folds over the visceral mass. The visceral mass contains digestive, nervous, excretory, reproductive, and respiratory systems. Most mollusks have an organ, the radula, that has teeth made of chitin. The mantle is the dorsal epidermis in mollusks. Most mollusks can secrete a calcareous shell for protection of its soft body. Mollusk fertilization occurs externally in aquatic varieties, but not terrestrial ones. In aquatic mollusks, the zygote hatches and produces a trochophore larva with cilia. Some larvae undergo a second larvae stage (veliger larvae) before becoming an adult.
Phylum Mollusca is divided into Caudofoveata, Solenogastres, Monoplacophora, Polyplacophora, Gastropoda, Cephalopoda, Bivalvia, and Scaphopoda.
Caudofoveata and Solengastres contain shell-less, worm-like animals in benthic marine habitats with calcareous spicules in their epidermis offering protection and the mantle cavity is smaller and there are no eyes, tentacles, or excretory organs. Caudofoveata has radula, but Solengastres do not have radula or gills. Caudofoveata has no foot.
Monoplacophora have one cap-like shell that covers the body, radula, digestive system, pairs of excretory organs, and a pair of gonad sex organs along with several gills.
Polyplacophora have many plates or chitons covering their body, a foot with suction capability, and a mantle that extends beyond the outer shell containing calcareous spines for protection. Multiple gills offer respiration in the mantle cavity. Blood from gills is collected in the heart and sent to the rest of the body in the hemocoel, an open circulatory system where the blood is contained in connected chambers around the organs rather than in individual blood vessels. The radula have teeth composed of magnetite, harder than any teeth in the biological world. The nervous system has ganglia at the anterior end with sensory structures, including photosensors that extend from the mantle to the channels of the upper shell called esthetes. Other sensory structures help find food. A pair of nephridia is used for excretion of nitrogenous wastes.
Bivalvia have two valves and includes the clams, oysters, muscles, scallops, geoducks, and shipworms. Bivalves range from microscopic to one meter in length and are found in marine and freshwater habitats. The two valves of the organism enclose the body, and the valves are fused together with hinge ligaments and teeth that keep the two halves aligned. The two shells contain an outer organic layer, a middle-prismatic layer, and a smooth nacreous layer, are joined at the umbo. Muscles of the bivalve help open and close the shell. The body of the bivalve is flat laterally, and the foot is wedge-shaped. Bivalves are filter-feeders and do not have radula. The mantle cavity is fused along the edges with openings for the foot and openings for the circulation of water in and out by siphons. During water intake, siphons capture food particles by the ctenidia gills and carried by cilia to the mouth. Excretion and osmoregulation are performed by a pair of nephridia. Eyespots and other sensory structures are located on the edge of the mantle, particularly noticeable in scallops. Three pairs of connected ganglia regulate activity of different body structures. The mantle functions to secrete the shell. Some bivalves can produce calcareous nacre material that can be used to produce pearls.
Gastropoda makes up more than half of molluscan species and are known for their "stomach foot," including snails, slugs, conchs, cowries, limpets, and whelks. These are found in aquatic marine and freshwater habitats, and terrestrial habitats. Some gastropods have shells and some do not. Gastropod bodies are asymmetrical and commonly have a coiled shell, which can be planospiral like garden snails or conispiral like conches. Some shell spirals are covered by the mantle. Gastropod embryonic development involves torsion, a unique process that occurs when the visceral mass, mantle, and shell of a gastropod rotate 180° in a counterclockwise direction relative to the head and foot. The torsion process produces the asymmetric shape of the developed gastropod. Gastropods have a foot used for crawling, a head with tentacles, eyes, and radula (tongue) used to obtain food particles. In aquatic gastropods, the mantle cavity encloses the gills (ctenidia), however, in land gastropods, the mantle works as the respiratory system and functions like a lung. Nephridia (kidneys) are located in the mantle cavity.
Cephalopoda are known as the head-foot animals and include octopuses, squids, cuttlefish, and nautiluses. Some cephalopods have shells and some have no shell. The nautilus has a multi-chambered spiral shell that fill with gas or water to regulate buoyancy. A siphuncle tube passes through the chambers and regulates the amount of water and gases in each chamber. Ammonites and other nautiloid shells are common in the fossil record. Squids and cuttlefish have smaller shells and that exist internally in the body and are known as squid pen and cuttlefish bone, respectively. Cephalopods can rapidly change their body color, particularly in squid and octopus, and is used for camouflage or mating signals. Pigment cells (chromatophores) are pigment cells in the skin can expand or contract to produce different color patterns. Underneath the chromatophores are iridophores and leucophores that can produce a greater variety of colors. Cephalopods are carnivores that have jaws and radula. Cephalopod nervous systems have image-forming eyes. Cephalopods have a closed circulatory system with blood vessels. The foot is lobed and divided into arms and tentacles. Suckers with rings are found on the arms and tentacles of octopuses and squid. Siphon tubes are used for locomotion by expulsion of water as propulsion. Gills (ctenidia) are located in the mantle cavity walls and large blood vessels each have their own heart. Nephridia organs are present in the mantle cavity for water balance and waste disposal. Cephalopods including octopuses and squid produce sepia or dark ink which contains melanin between the gills and can be released into the water to produce a smoke screen to hide from predators. Cephalopods are dioecious as member species mate and the female lays eggs that hatch and develop without a larva stage.
Scaphopoda are known as boat feet, tusk shells, or tooth shells that are usually buried in the sand with only the anterior opening exposed. These have a single conical shell open on both ends. There is a head, a mouth with radula, and surrounded by tentacles with bulbs that can catch prey. Scaphopods also have a foot, but there is no ctenidia (gill), but the mantle cavity serves as a tube that extends to both ends and allows for respiration.
Phylum Annelida includes the segmented worms found in marine, freshwater, and terrestrial environments. Annelids require water or humidity for survival in terrestrial environments. Annelids are called segmented worms because their bodies have metamerism or true segmentation. Marine annelids, earthworms, and leaches are annelids and some are parasitic or symbiotic. Annelids have bilateral symmetry with ring-like segmentation. The metameric segmentation where several internal and external morphological features are repeated in each segment and the organism can become bigger by adding new segments and improving their movement. The annelid body contains a head, body, and pygidium (tail). As the annelid develops, segments behind the head appear sequentially from growth from the tail, or teloblastic growth. In the Oligochaetes, the clitellum is a reproductive structure that generates mucus to aid sperm transfer and produces a cocoon, where fertilization occurs. The epidermis is protected by a cuticle and muscles are located inside the epidermis. Setae are chitinous bristles set or anchored in the epidermis and connected to muscles. In the polychaetes, setae are born on paired appendages called parapodia. Most annelids have a digestive system and feeding mechanisms vary, such as with filter-feeders, tentacles, jaws, or pharynx. Earthworms collect food as they burrow through the soil. Earthworm digestive tracts include a mouth, muscular pharynx, esophagus, crop, and muscular gizzard. The gizzard leads to the intestines, which ends in the anal opening in the terminal segment. Most annelids have a closed circulatory system of blood vessels that run parallel to the digestive tract and capillaries that connect to individual tissues. The vessels are connected by transverse loops in every segment. Some polychaetes and leaches have open circulatory systems where blood vessels open into a hemocoel. Annelid respiration occurs as gas exchange across the body surface. In polychaetes, vascular parapodia serve as respiratory structures. Excretion occurs with a metanephridia tubule waste disposal system in every segment. A ring of fused ganglia around the pharynx serve as the nervous system. A ventral nerve cord has nodes in each segment. Annelids can be monoecious with permanent gonad sex organs as in earthworms or leaches or dioecious with temporary gonads as in polychaetes, but cross-fertilization is preferred. In polychaetes, fertilization is external, while in oligochaetes, fertilization is internal. Polychaetes can regenerate and some can reproduce asexually. Phylum Annelida classification contains the polychaetes (many bristles), and the oligochaetes (few bristles; earthworms, leaches).
Superphylum Ecdysozoa includes the Nematodes, Tardigrades, and Arthropods. The most distinguishing feature of this superphylum is the cuticle, or exoskeleton that protects these animals from dehydration, predators, and other environmental factors. All members of superphylum Ecdysozoa also shed their old exoskeleton after a new one grows underneath it by secretion.
Phylum Nematoda animals are triploblastic (endoderm, mesoderm, ectoderm), bilaterally symmetrical, are pseudocoelomates, and show tubular morphology and a circular cross section. Some nematodes are parasitic. Nematodes live in all habitats and are abundant through out the world. The nematode body is cylindrical and has a digestive system with a mouth and anus. The mouth connects to a muscular pharynx and intestine, rectum, and anal opening at the other end. The epidermis can be a single layer of cells or a syncytium, a multinucleated tissue that is formed by the fusion of many single cells. The cuticle of nematodes is rich in collagen and a polymer called chitin, which forms a protective armor outside the epidermis. The cuticle extends into both ends of the digestive tract, the pharynx, and rectum. In the head, the anterior mouth opening is composed of three (or six) lips and teeth derived from the cuticle in some species. Some nematodes also have rings, head shields, or warts, but the rings do not represent segmentation. Muscle attachment in nematodes is unique in that they have a longitudinal layer only and are directly attached to the dorsal and ventral nerve cords, which creates a strong muscular contraction that results in a whiplike body movement. In nematodes, wastes are released directly across the body wall or osmoregulation and salt balance are performed by excretory cells (renette cells) or glands that can be connected to canals that release wastes through pores. Nematodes have four longitudinal nerve cords that run the length of the body in dorsal, ventral, and lateral positions and these cords fuse at the anterior end, to form a pharyngeal nerve ring around the pharynx, which acts as the head ganglion or brain of the nematode roundworm. A similar fusion occurs at the other end of the body. Nematode reproduction methods can vary, from monoecious, where one organism has both sex organs, to dioecious, where each organism has either one sex organ or the other, to parthenogenic, with no male partner. Fertilization is internal, and the embryo develops soon after fertilization. Parasitic nematodes include guinea worms, hookworms, Trichina worms, Ascaris, Dirofilaria, and Wuchereria. These are a threat to other animals and humans from contaminated water.
Phylum Tardigrada are known as slow steppers or water bears and are small animals living in marine, freshwater, and wet terrestrial environments. Tardigrades have a plump body, short legs, and claws. A chitinous cuticle covers the body surface and can be divided into plates. Tardigrades are known for their ability of cryptobiosis, a state of resistance to many environmental difficulties and extreme conditions, including temperature, pressure, dryness, and radiation. Tardigrades can pause their body functions for years and survive with low levels of water because of specialized proteins that replace water in their cells and protect their cellular DNA structure from damage. Tardigrades have cylindrical bodies and four pairs of legs with claws. The first three pairs of legs are used for walking, while the rear pair of legs are used for attachment to a surface. The mouth is connected to a muscular pharynx and salivary glands as they feed on plants, algae, or small animals. Muscle cells extend from the epidermis to the legs and help in movement. The hemocoel is the major body cavity where tubules remove metabolic waste and transports them through the body. The dorsal brain is connected to the ventral nerve cord with segmented ganglia in the appendages. Sensory structures include eyespots, cilia, or bristles. Most tardigrades are dioecious and each individual tardigrade has a certain gonad, male or female. Fertilization is external and development produces a fixed number of cells that enlarge but do not multiply.
Phylum Arthropoda or arthropods, are coelomates (internal fluid chamber) with a chitinous exoskeleton and jointed appendages. Arthropods go through the process of molting and ecdysis (shedding of exoskeleton). Arthropods are protostomic in their life cycle. Arthropod means jointed feet. Arthropod characteristic morphology includes a segmented body both structurally and functionally, jointed appendages, and waterproof exoskeleton made of chitin. Arthropods live in terrestrial, aquatic, and aerial habitats. Arthropoda includes trilobites, horseshoe crabs, spiders, scorpions, ticks, mites, daddy long legs or harvestmen, millipedes, centipedes, crustaceans, crabs, lobsters, crayfish, and insects of six legs. Trilobites are extinct and can be seen in the fossil record. Arthropod characteristics include jointed appendages, body segmentation, and chitin exoskeleton. Functional body regions (tagmata) include the head, thorax, abdomen, cephalothorax and abdomen, or head and trunk. The arthropod central cavity is the hemocoel blood cavity and fluids are moved by contraction of regions by the tubular dorsal blood vessel. Some arthropods use green glands or Malpighian tubules for waste excretion. The nervous system is distributed along the segments and a ventral nerve cord. Arthropod respiratory systems vary. Tubes or trachae branch through the body to tracheoles that perform gas exchange. Trachea open to the surface of the cuticle via apertures called spiracles. Aquatic crustaceans use gills, terrestrial chelicerates have book lungs, and aquatic chelicerates use book gills. Book lung tissue looks like the pages of a book. The gills of crustaceans exchange gases with the surrounding water.
The cuticle is the hard covering of the arthropod, and it has two layers: the epicuticle is a thin and water resistant outer layer with no chitin, while the layer beneath is chitinous procuticle, with an upper and lower layer. The exoskeleton is made of chitin bound with a protein. The procuticle is flexible, lightweight, and protects against dry environment. The arthropod grows by shedding its exoskeleton by molting and ecdysis (stripping).
Subphylum Chelicerata includes horseshoe crabs, sea spiders, spiders, mites, ticks, scorpions, and harvestmen. Most of these are terrestrial, but some are aquatic freshwater and marine. The body of chelicerates are divided into two tagmata body parts: prosoma and opsithosoma, which are similar to the cephalothorax and abdomen. A distinct head is usually unnoticeable. The chelicerae is the first pair of appendages with claw-like mouthparts. The second pair of appendages is called the pedipalps and can be used for sensing or food manipulation. Some species have an extra pair of appendages between the chelicerae and pedipalps called ovigers, used for grooming and carrying eggs. The nervous system in chelicerates contains a brain and two ventral nerve cords. Chelicerates are dioecious with separate sexes for reproduction and use external and internal fertilization.
Subphylum Myriapoda includes the arthropods (centipedes and millipedes) with numerous legs, but usually between 10 to 750. Myriapoda are found in humid terrestrial environments such as moist soils and decaying biomaterial such as leaves. Centipedes have one pair of legs per segment connected laterally, mouthparts, and somewhat flattened. Centipedes are predators and have poison claws to capture prey. Millipedes have two pairs of legs per diplosegment attached underneath, rounder in cross-section than centipedes, and are herbivores.
Subphylum Crustacea are dominant in freshwater and marine environments. Krill, shrimp, lobster, crabs, and crayfish are types of crustaceans. Terrestrial crustaceans include pill bugs, roly-polies, potato bugs, and isopods. Crustaceans have two pairs of antennae, mandibles as mouthparts, two branched appendages so that their legs are formed in two parts: endopods and exopods. Insects have one branched or unibranched appendages. In most crustaceans, the head and thorax are fused together to form the cephalothorax, which is covered by a plate called a carapace. The body plan has two tagmata: cephalothorax and abdomen. A chitinous exoskeleton is shed by molting. Crustaceans have an open circulatory system where blood is pumped into the hemocoel by the heart. Most crustaceans are dioecious, but barnicles may be hermaphrodites. Serial hermaphroditism is where the gonad can switch from producing sperm to ova in some species. Larval stages are seen in early development aquatic crustaceans. Crustaceans have a brain formed by the fusion of the first three segmental ganglia and two compound eyes. A ventral nerve cord connects additional ganglia. Most crustaceans are carnivorous, but some are herbivorous or even parasitic.
Subphylum Hexapoda is the largest class of arthropods and includes the insects. The common feature is six legs or three pairs of legs. Hexapod bodies are divided into three tagmata: head, thorax, and abdomen. Mouthparts are on the head and the thorax has three pairs of jointed appendages, along with wings. Pterygotes are the winged insects. THe head has an upper lip and lower lip that serve as mouthparts along with sensory antennae, compound eyes, ocelli (simple eyes), and sensory hairs. The abdomen has 11 segments along with reproductive parts. The evolution of wings is difficult to explain for established scientists. Many insects develop by metamorphosis from larvae to adult, such as with ants, beetles, flies, and butterflies. Some insects work in groups or colonies and use pheromones, external chemical signals, to communicate with each other.
Phylum Echinodermata and Chordata, which includes humans, belongs to the superphylum Deuterostomia. Protostomes and deuterostomes are named based on which opening of the archenteron, or gut tube, develops first. In deuterostomes, the mouth develops as a secondary structure opposite the blastopore, which becomes the anus. In protostomes, or mouth first, the first embryonic opening becomes the mouth, and the second opening becomes the anus.
More differences between protostomes and deuterostomes include: type of embryonic cell division or cleavage, mode of formation of the coelom of the embryo: Protostomes have spiral mosaic cleavage while deuterostomes have radial cleavage. In deuterostomes, the endodermal lining of the archenteron usually forms buds called coelomic pouches that expand and ultimately destroy the embryonic blastocoel, the cavity within the blastula and early gastrula to become the embryonic mesoderm, the third germ layer. This happens when the mesodermal pouches become separated from the invaginating endodermal layer forming the archenteron, then expand and fuse to form the coelomic cavity. The resulting coelom is called an enterocoelom. The archenteron develops into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the blastopore of the gastrula. The blastopore forms the anus of the alimentary system in the juvenile and adult forms. Cleavage in most deuterostomes are also indeterminant, meaning that the developmental fates of early embryonic cells are not decided at that point of embryonic development.
The deuterostomes consist of two major clades: the Cordata and the Ambulacraria. The Chordata includes the vertebrates and two invertebrate subphyla (urochordates and cephalochordates). The ambulacraria include the echinoderms and the hemichordates.
Most invertebrates do not have a bony vertebral endoskeleton, or bony cranium. However, Echinodermata produce tiny skeletal bones called ossicles that create an endoskeleton, or internal skeleton covered by an epidermis.
Echinodermata is named for their prickly skin, marine, bottom-dwelling habitats including sea stars, sea cucumbers, sea urchins, sand dollars, and brittle stars. Echinoderms have pentaradial symmetry with arms around a central axis. They have an endoskeleton made of calcareous ossicles, or small bony plates, covered by the epidermis. In some species, the spines are surrounded by tiny stalked claws called pedicellaria, which help keep the animal's surface clean of debris, protect papulae used in respiration, and sometimes aid in food capture.
The endoskeleton is produced by dermal cells, which also produce several kinds of pigments. Sea stars have finger like projections (papilae) of dermal tissue extend through the endoskeleton and function like gills. Some cells are toxic. Each arm contains different structures: digestive glands, gonads, and tube feet. Every arm bears two rows of tube feet on the oral side along an external ambulacral groove. The tube feet assist in locomotion, feeding, and chemical sensations, as well as serve to attach some species to the substratum.
Echinoderms have a unique ambulacral water vascular system, derived from the coelom body cavity. The water vascular system contains a central ring canal and radial canals that extend along each arm. Each radial canal is connected to a double row of tube feet, which project through holes in the endoskeleton, and function as tactile and ambulatory structures. The tube feet can extend and retract based on the volume of water present in the system of that arm, allowing the animal to move and also allowing it to capture prey with their suckerlike action. Individual tube feet are called ampullae. Sea water enters the system through the aboral madreporite and passes to the ring canal through a short stone canal. Water circulating through these structures facilitates gaseous exchange and provides a hydrostatic source for locomotion and prey manipulation. A hemal system, consisting of oral, gastric, and aboral rings, as well as other vessels running roughly parallel to the water vascular system, circulates nutrients. The water vascular and the hemal system transport nutrients and gases along with the visceral body cavity.
The nervous system of echinoderms has a circumoral nerve ring at the center and five radial nerves extending outward along the arms. Also, several networks of nerves are located in different parts of the body. The mouth opens to an esophagus to a stomach. Digestive glands (pyloric caceca) in each arm, running dorsally along the arms and the reproductive glands below them. Then digested food is transferred to the anus. Podocytes are cells specialized for ultrafiltration of bodily fluids are located near the center of the echinoderm disc. These podocytes are connected by an internal system of canals to the madreporite, where water enters the stone canal. The adult echinoderm usually has a large, fluid filled coelom. Cilia aid in circulating fluid within the body cavity, and lead to the fluid filled papulae, where the exchange of oxygen and carbon dioxide take place, as well as the secretion of nitrogenous waste such as ammonia, by diffusion. Echinoderms are dioecious, but males and females are indistinguishable aside from their gametes, which are released into water and fertilization is external. Early larval stages of echinoderms have bilateral symmetry and the radially symmetric adult forms a cluster of cells in the larvae. Echinoderms may also reproduce by asexual fragmentation.
The Phylum of Echinoderms are divided into five classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lillies and feather stars), and Holothuroidea (sea cucumbers). Sea stars have thick arms that extend from a central disk and various organs branch into the arms. Each arm has eyes and sensory tentacles. Sea stars use their rows of tube feet for attaching to surfaces and grasping prey. Sea stars are mostly carniverious and have two stomachs. They use their arms to open bivalve molluscs. Brittle stars (snake-tails) have long, thin, flexible arms that extend from their central disk and are used for locomotion and grasping prey. Sea urchins and sand dollars do not have arms, but are hemispherical or flattened with five rows of tube feet that extend through five rows of pores in a continuous internal shell called a test. Their tube feet help keep their body surface clean as they feed on algae. Sea lilies and feather stars are Crinoids. Sea lilies are sessile and attached to a stalk, but feather stars can move with their leglike cirri. Sea lilies and feather stars are both suspension feeders and their arms are used to catch food. Sea cucumbers are the only echinoderms that are bilaterally symmetrical as adults, as they lie horizontally. Tube feet are little or none, and there is a single gonad. Respiratory trees are gill-like structures that branch from the gut and muscles pump water in and out. Tentacles cluster around the mouth, and eat detritus or suspension organisms.
Phylum Chordata has five key features: a notochord, a dorsal hollow nerve cord, pharyngeal slits, a post-anal tail, and an endostyle/thyroid gland that secretes iodinated hormones. Chordata contains vertebrate classes and two invertebrate classes: Urochordata and Cephalochordata.