Vertebrate Design by Owen Borville BIO 29 November 27, 2024 Biology
The design of vertebrate animals includes animals that have a backbone and a skeleton by definition.
Phylum Chordata (Chordates) have five key characteristics: a notochord, a dorsal hollow (tubular) nerve chord, pharyngeal gill arches or slits, a post-anal tail, and an endostyle /thyroid gland.
The notochord, which chordates are named, is a flexible, rod-shaped mesodermal structure that is found in the embryonic stage of all chordates and some of the adult chordates. The notochord provides rigidity, skeletal support, and flexibility. The notochord is located between the digestive tube and the nerve chord.
The dorsal hollow nerve cord rolls into a hollow tube during development and is located dorsally from the notochord.
Pharyngeal slits are openings in the pharynx, or the region just posterior to the mouth that extend to the outside environment. These slits allow for the exit of water in aquatic environments that enters from the mouth. Some chordates use the slits for filtering food from the water.
The post and tail is a posterior elongation of the body, extending beyond the anus. The tail contains skeletal elements and muscles, that allow for locomotion in aquatic environments and also for terrestrial varieties helps with balance, courting, and signaling.
Two types of chordates are invertebrates: Cephalochordata and Urochordata
Cephalochordata members have a notochord, dorsal hollow tubular nerve chord, pharyngeal slits, endostyle/thyroid gland, and post-anal tail in the adult stage. Lancets are a few centimeters long and are found buried in the sand at the bottom of warm, tropical seas. Cephalochordates are suspension feeders. Water current is created by cilia in the mouth and the water is filtered through oral tentacles. The pharyngeal slits filter out food particles. The filtered water collects in a gill chamber called the atrium and exits through the atriopore.
Urochordata (or tunicates) have a cellulose-like carbohydrate material called tunic which covers the outer body. Adult tunicates do not have a notochord, dorsal hollow nerve cord, or a post-anal tail, but do have pharyngeal slits and an endostyle. The larval form contains all five of these structures, but three disappear upon adult form. Most tunicates are hermaphrodites. Adult tunicates can be either solitary or colonial. Most tunicates are suspension feeders.
Subphylum Vertebrata (Craniata) features the cranium, a bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and face bones. These are bilaterally symmetrical animals that include the jawless hagfishes, lampreys, and all vertebrates. Hagfish have a cranium but lack a backbone. Members of Craniata/Vertebrata have the five characteristic features of chordates. However, all vertebrates have a vertebral column, composed of vertebrae, which are a series of irregularly shaped bones joined together to form a backbone. The vertebrates are the largest group of chordates.
Jawless fishes are known as the superclass Agnatha. Class Myxini contains the hagfishes, lenticular scavengers that live on the bottom of the ocean floor and eat any animals available. Hagfish have poor eyesight, but have sensory organs on their mouth and have teeth that can pull flesh from prey. The gills are used for respiration and their body surfaces contain slippery mucus substances produced inside their skin that allow them to evade predators. Their bodies are also flexible and they can tie their bodies into a knot. Hagfishes have a cartilaginous skull and a fibrous and cartilaginous skeleton, while the notochord runs the length of the body and provides support.
Class Petromyzontida includes the lampreys, which are similar to hagfish in size and shape. However, lampreys have extrinsic eye muscles, two or more semi-circular canals, a cerebellum, and cartilaginous vertebrae above the notochord. Lampreys have a dorsal tubular nerve cord with a well differentiated brain, a small cerebellum, and 10 pairs of nerves. Lampreys have one or two fleshy dorsal fins, and teeth around its mouth. Many lampreys have a parasitic stage in their life. Lampreys live in coastal and freshwater environments around the world in temperate climates. Lamprey eggs fertilize externally.
Gnathostomes, or jaw-mouths, are vertebrates with true jaws: hinged structures attached to the cranium that allow for grasping and tearing food. However, evolutionists cannot explain how these animals "suddenly" obtained jaws in the fossil record. In addition to jaws, gnathostomes have paired appendages, teeth, specialized embryonic and skeletal tissues.
Class Condrichthyes are the cartilaginous fishes such as sharks, rays, skates, and sawfishes. Condrichthyes are jawed fishes with paired fins and a skeleton made of cartilage. They are mostly marine with a few freshwater varieties and are carnivorous. Shark skin has tooth-like scales called placoid scales. Some sharks and skates are suspension feeders. The sawfish front looks like a double-edged saw and the rostrum, or pointed snout, is covered with electrosensitive pores that help the animal find prey. Electroreceptors called ampullae of Lorenzini allow sharks to detect electromagnetic fields that are produced by all living things, including their prey. Sharks and most fishes and amphibians have a row of sensory structures called the lateral line, which is used to detect movement and vibration in the surrounding water and is functionally similar to hearing in land vertebrates. The lateral line is visible as a dark stripe along the length of the body. Sharks must swim continuously to maintain buoyancy with the water.
Sharks reproduce sexually and eggs are fertilized internally in the female. In ovoviviparous sharks, the eggs hatch in the uterus and the newborn hatched sharks are alive and fully functional. Some sharks, however, are oviparous, that lay eggs externally from the body that are protected inside a pouch case that has tentacles that grab seaweed to protect the eggs. Some sharks are viviparous, or give live birth after fertilization and embryo development occur inside the mother's body.
Chondrichthyes usually have a flattened body, unequal sized fin lobes, with the tail vertebrae extending into the larger upper lobe, paired pectoral and pelvic fins, exposed gill slits, and intestine with a spiral valve that condenses the length. They also have three pairs of semi-circular canals, excellent senses of smell, vibration, vision, and electroreception.
Rays and skates are similar to sharks but their bodies are flatter, their pectoral fins are enlarged and fused to their head, and their gill slits are located on their ventral surface. Rays and skates have cartilaginous skeletons just like sharks and most are marine and live on the seafloor.
Holocephali (chimaeras or ratfish) have a diphycercal tail with equal sized fin lobes, and tail vertebrae located between them. Holocephali lack scales and have teeth like grinding plates used to feed on mollusks and other invertebrates. Chimaeras have four pairs of gills covered by an operculum.
Osteichthyes are the bony fishes and are characterized by a bony skeleton. Most bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that produce and maintain a calcium phosphate matrix. However, sturgeon and paddlefish have mostly cartilaginous skeletons. The skin of bony fishes usually contains overlapping scales and skin mucus aids in swimming dynamics and osmoregulation. All bony fish use gills to breathe, as water is drawn over the gills that are located in chambers covered and ventilated by a protective flap called an operculum. Many bony fishes also have a swim bladder filled with gas as a pouch from the gut to control buoyancy and gases of the swim bladder are exchanged with the blood.
Two classes of bony fishes are ray-finned (Actinopterygii) and lobe-finned (Sarcopterygii). Ray-finned fishes include tuna, bass, trout, salmon and have a fan of slender bones that supports their fins. The lobe-finned fishes are fleshy and lobed, supported by bones, and include the lungfish and coelacanths, which are living fossils once thought to be extinct.
Amphibians are vertebrate tetrapods with four limbs, including frogs, salamanders, and caecilians. Most amphibians undergo a life cycle from water to land. Most amphibians have moist, permeable skin made possible by mucus glands. Amphibians intake more water from their skin than by drinking. Amphibians also breathe through their skin, in addition to their lungs and mouth. Amphibians are carnivorous, aided by a sticky tongue used to catch prey, along with teeth and jaws. Amphibians have teeth on both sides of the jaw or on one side of the jaw. Some amphibians have multiple rows of jaws. Some amphibians have vomerine in the roof of the mouth. Amphibians have eyes for vision and can see color. Ears allow amphibians, particularly frogs and toads, to communicate with each other by vocalization. Different sections of the inner ear can detect different frequencies of sounds below and above 10,000 hertz. The papilla amphibiorum can detect below 10,000 hertz while the papilla basilaris can detect above 10,000 hertz such as mating calls. The operculum in the ear can detect seismic signals. Amphibian classes are divided into Urodela (tailed), Anura (tail-less), and Apoda (legless).
Salamanders (Urodela) include both aquatic and terrestrial types with four limbs and a tail. Most salamanders do not have lungs and breath through their skin or external gills. Some land salamanders have lungs. Some salamanders have lungs and gills. Salamanders reproduce using internal fertilization of the eggs. Sperm of the males is deposited in a packet called a spermatophore and picked up by the female. Eggs are hatched after being laid in most salamanders (oviparous). Aquatic salamanders lay their eggs in water, and the eggs hatch into larvae. Land salamanders lay their eggs in nests, and are guarded by the mother. These larvae gain adult form before hatching. The Mexican axolotl stay in the larvae stage.
Frogs (Anura) are found on all continents except Antarctica. Frogs vary greatly in size from millimeters to centimeters. Their hind legs are capable of jumping in order to catch prey. Tree frogs have hands specialized for grabbing branches as they climb. Flying frogs in tropical environments can move with their webbed feet. Their skin acts as camouflage. Frogs and salamanders have chemical defense systems in their skin glands. Frog eggs are fertilized externally in moist environments. Some frogs do not perform parental care of eggs, while others carry their eggs on their body or place them on a leaf. The life cycle of most frogs involves the larval stage and the metamorphosis into adult stage. However, some frogs develop directly from eggs to adult stage. The larval frog, or tadpole, is a filter-feeding herbivore. Tadpoles have gills, a lateral line system, longfinned tails, and no limbs. After metamorphosis, gills, tail, and lateral line disappear and four limbs develop. The rest of the body becomes larger and more mature in adult form and allow the adult to move onto land.
Apoda (Caecilians) are a group of amphibians that have no limbs and resemble earthworms. There are also folds of skin that resemble the segments on earthworms. However, unlike earthworms, caecilians have teeth in both jaws, and eat organisms in the soil, including earthworms. Caecilians can burrow in the soil or swim, but have poor eyesight. Caecilians have a single lung and breathe through their skin. Caecilians are found in tropical environments of the world of Africa, Asia, and South America. Males have copulatory structures to transfer sperm into the female, and fertilization is internal. Some caecilians are oviparous (eggs hatch outside the mother's body), but most caecilians produce live births.
Reptiles
The features of reptiles include land-based eggs, which has four extraembryonic membranes: the yolk sac, the amnion, the chorion, and the allantois. The chorion and amnion develop from folds in the body wall, and the yolk sac and the allantois are extensions of the midgut and hindgut respectively. The amnion forms a fluid-filled cavity that provides the embryo with its own aquatic environment. The eggs of reptiles have shells. An amniote embryo was then enclosed in the amnion, which was then enclosed in an extra-embryonic coelom contained within the chorion.
An amniote is an animal whose embryo develops in an amnion and chorion and has an allantois. Amniotes include mammals, birds, or reptiles. The amniotic egg is the key characteristic of amniotes. The shell of the egg provides protection while allowing exchange of carbon dioxide and oxygen gas. The albumin, or egg white, outside the chorion provides the embryo with water and protein, while the egg yolk provides nutrients for the embryo. Blood vessels in the yolk sac transport yolk nutrients to the circulatory system of the embryo. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg's external environment. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. The amnion protects the embryo from mechanical shock and supports hydration.
Some reptiles are tetrapods, having four legs, but some reptiles, like snakes, do not have any legs. Reptile eggs are calcareous or leathery and all reptiles lay eggs on land, even aquatic reptiles return to land to lay eggs. Reptiles usually reproduce sexually with internal fertilization. In some reptile species, eggs stay in the mother until they are ready to hatch. In other species, babies are born alive.
Many reptiles have scaly skin containing the protein keratin and waxy lipids, which help reduce water loss from the skin. Some reptiles have claws or nails or hair. Reptiles cannot use their skin for respiration, and instead use lungs. Reptiles grow throughout their lives and shed their skin regularly as it helps them grow and to remove parasites. Reptiles use their muscles to ventilate their lungs and inhale air by reducing pressure. Most reptiles are ectotherms, and their body heat comes from the environment, and their bodies don not produce heat themselves. Therefore, reptiles can go long periods without food because they don't need as much energy from food to heat their body. The reptile can survive even longer without food by being inactive and conserving energy during colder weather, called brumation.
Crocodilia is a type of reptile that includes the alligators and crocodiles that live in tropical and subtropical climates. They can live in freshwater, saltwater, rivers, lakes, and stay in the water most of the time. Their legs help them swim in the water and also walk on land. Some species of crocodiles can even gallop.
Spenodontia are known commonly as tuatara named for the crest on its back and has a skull and vertebrae. It has four legs and a tail. Length is up to 80 centimeters. Tuataras have no external ears. They have a third eye in between the two main eyes which is not visible in adults. Their jaws have two rows of teeth in the upper jaw and a single row on the bottom jaw. Tuataras are found in New Zealand.
Squamata are scaly lizards and snakes found on all continents except Antarctica. Lizards differ from snakes in having four legs to none. Snakes do not have eyelids and external ears, while lizards do. Lizards include the small chameleons and geckos, and the large Komodo dragon. Lizards usually have spines, crests, and frills, and many are brightly colored. Chameleons can change their skin color as pigment is redistributed in the chromatophore cells in the skin. They change color for camouflage to help avoid predators and for social signaling. Lizards have color vision and can focus their eyes by changing the shape of the lens. The eyes of chameleons can move independently. Some lizards have a parietal third eye like the tuatara. Both lizards and snakes use their tongue to sample the environment and Jacobson's organ in the roof of the mouth helps identify a sample. Most lizards are carnivorous, but some are herbivores. Lizard tongues are long and sticky and enabling the catch of prey. Some lizards have venom for self defense. Special features of the jaw help ingestion of large prey in reptiles and all snakes are carnivorous.
Snakes range from 10 centimeters to 10 meters in length. All snakes are legless and have a single lung. Most snakes have a flexible and rotational skull. The lower jaw does not have a bone or ligament connection, enabling a great expansion for eating larger animals. Most snakes are non-venomous and swallow their prey whole. Venom helps capture prey in those that have it and in digestion. Snakes have no eyelids, but do have a protective transparent scale. Some snakes can see ultraviolet light, which helps in finding prey. Snakes have inner ears and some snakes can detect infrared heat emissions from prey with a sensory organ between the eyes and nostrils. Snakes also use Jacobson's organ for sensing.
Testudines include the turtles, terrapins, and tortoises, which are characterized by a bony or cartilaginous shell. The shell in turtles is not just an outer covering, but is built into the skeletal system. The dorsal shell is called the carapace and includes the backbone and ribs, while the ventral shell is called the plastron. Both shells are covered with keratinous plates or scutes, and the two shells are held together by a bridge. Some turtles can hide their head and legs under the shell by way of a hinge in the shell. In the two groups of living turtles, the Cryptodira retracts its neck in a vertical S-curve. Pleuroida retracts their necks with a horizontal curve, folding the neck to the side. Turtles are ectotherms like most reptiles and depend on heat from the environment. All turtles lay their eggs on land, but many turtles live in or near water. Parental care is non-existent. Turtle size can range from 8 centimeters to 200 centimeters.
Birds have common features including feathers and wings. Birds are endothermic so that they produce their own body heat and maintain a constant internal temperature so that they can remain active daily and yearly throughout the world, have faster nerve impulses, increased muscle strength, and greater physical endurance. Birds are homeothermic in that they can maintain an elevated and constant body temperature. Bird temperature is higher than most mammals because birds produce much energy from their active lifestyle. Bird feathers also help retain internal body heat.
Birds have two main types of feathers: contour feathers and down feathers. Contour feathers have a number of parallel barbs that branch from a central shaft. The barbs have microscopic branches called barbules linked together by small hooks and this structure helps the feather to be strong, flexible, and have a complete, uninterrupted surface. The barbules of down feathers do not interlock, making these feathers good for insulation by trapping air between the spaces of the barbules of adjacent feathers, reducing heat loss.
Feathers also provide the necessary lift for flight while the flexibility of the feathers reduces drag during flight on the wings. The asymmetrical curved shape of the wing allows lift to occur during flight. Flight feathers are divided into primary and secondary feathers. Primary feathers are located at the tip of the wing and provide thrust as the bird moves its wings downward, using major muscles. Secondary feathers are located closer to the body in the forearm of the wing, and provide lift. Contour feathers on the body help reduce drag produced by wind resistance during flight, creating a smooth surface for air to move over while preventing turbulence.
Flapping of the wing is made possible by chest muscles, including the pectoralis major muscles that move the wings downward, while the supracoracoideus muscles move the wings upward. The flight muscles are rich in oxygen-storing myoglobin. The collarbones are attached, forming the furcula or wishbone, which is flexible but supportive of flight.
Flying birds must have a low body weight and the heaviest birds are usually flightless, like the ostrich. Pneumatic bones in birds are hollow, and not filled with tissue, but have "cross struts" in between the bones called trabeculae for structural support. Not all bones in the bird skeleton are pneumatic, but many are including the skull. Beaks are made of keratin, a lighter material that helps reduce weight. The lack of a urinary bladder also reduces weight. Birds have a cloaca, an external body cavity where the intestinal, urinary, and reproductive organs empty and water can be recycled into the blood. Uric acid is converted to solid form and released with other solid waste so that the bird does not need to store extra liquid urine in a bladder, and helping to reduce weight. Female birds also commonly have only one ovary instead of two as in other animals, and therefore weight is reduced.
The respiratory system of birds allows for the high metabolic rate required for flight. Air spaces in the pneumatic bone are sometimes connected to air sacs in the body cavity, which replace coelomic fluid and lighten the body. These air sacs are also connected to the path of air flow through the birds body, and function in respiration. Airflow in bird lungs is unidirectional, unlike in two directions in mammals. Gas exchange occurs in air capillaries or microscopic air passages within the lungs. The arrangement of air capillaries in the lungs creates a cross-current exchange system with the pulmonary blood. In a counter-current system, the air flows in one direction and the blood flows in the opposite direction, producing a favorable diffusion gradient and creating an efficient means of gas exchange. This effective oxygen-delivery system supports higher metabolic activity. Ventilation is provided by the parabronchi expandable lungs within the air sacs located among the visceral organs and skeleton. A syrinx voice box resides near the junction of the trachea and bronchi.
Other unique feature of birds are the elongated, s-shaped neck, and short tail connected to the vertebrae. Bird vertebrae structure allows for more flexibility than mammal necks. Birds have a thin epidermis and no sweat glands. A specialized uropygial gland, or sebaceous "preening gland" found at the dorsal base of the tail. This gland is essential to continuous preening as an oily substance is produced that helps birds waterproof their feathers and keep their feathers flexible. Some birds do not have this gland, but have special feathers that produce a powdery down that has a similar effect as the gland.
Birds have 12 pairs of cranial nerves like mammals and a large cerebellum with optic lobes. Birds also have a single bone in the middle ear called the columella. Birds have a closed circulatory system with two atria and two ventricles with a right-bending aortic arch (left in mammals), along with nucleated red blood cells (enucleated in mammals). Birds are unique in the animal world with their large brains, strong senses, vocalization ability, and tool use.
Mammals are vertebrate animals that have hair and mammary glands. Most mammals are placental, except monotremes and marsupials. Certain other features of anatomy are characteristic of mammals. Hair in mammals is composed of the protein keratin and this hair has many important functions. Since mammals are endothermic and produce their own heat, hair on mammals helps retain heat generated by metabolic activity as insulation. Hair also serves as a sensory mechanism by specialized hairs called vibrissae or whiskers, which are attached to nerves that transmit information about tactile vibration produced by sound sensation. Hair can also serve as protective coloration or social signaling, when an animals hair stands up or outward to scare predators.
The skin of mammals contains several types of secretory glands not found in birds. Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin, providing water resistance and lubrication for hair and these are located on most of the body. Eccrine glands produce sweat, or perspiration, which is mainly composed of water, but also contains metabolic waste products, and sometimes compounds with antibiotic activity. In most mammals, eccrine glands are limited to certain areas of the body, and some mammals do not have them at all. In primates and humans, sweat glands are located across most of the body surface and these help regulate body temperature. Apocrine glands, or scent glands, secrete substances that are used for chemical communication, as do skunks. Mammary glands produce milk that is used to feed newborn babies. Evolutionists have difficulty explaining the origin of mammary glands.
Mammal skeletal systems feature a lower jaw with only one bone, the dentary, and the jaw hinge connects the dentary to the flat part of the temporal bone in the skull. Other vertebrate jaws have several bones. In mammals, the quadrate and articular bones are in the middle ear. The stapes, incus, and malleus are bones of mammal ears going from inward to outward.
The adductor muscles that close the jaw are the two major muscles in mammals: the temporalis and the masseter. These muscles allow up and down and side to side movements of the jaw, so that chewing is possible, which is a unique characteristic of mammals. Most mammals have heterodont teeth meaning different types and sizes of teeth, such as incisors, canines, and molars. Most mammals are also diphyodonts meaning that they have two sets of teeth in their life: baby teeth and permanent teeth. A few mammals and other vertebrate animals replace teeth throughout their lives.
Mammals are like birds in that they have a four-chambered heart. Mammals also have a specialized group of cardiac cells or fibers located in the walls of their right atrium called the sinoatrial node, or pacemaker, which determines the rate at which the heart beats. Red blood cells in mammals do not have nuclei, but other vertebrates have nucleated red blood cells.
The kidneys of mammals have a nephritic loop which allows mammals to produce urine with high concentrates of solutes, even higher than blood. A urinary bladder is present in all mammals.
The skulls of mammals have two occipital condyle bones at the base of the skull that associate with the first vertebrae and a secondary palate at the rear of the pharynx that separates the pathways of swallowing and breathing. Turbinate bones or conchae are located along the sides of the nasal cavity that help warm and moisten air as it is inhaled. The pelvic bones are fused in mammals, and there are usually seven cervical vertebrae. Mammals have moveable eyelids and fleshy external ears called pinnae. Mammals have a muscular diaphragm.
Brains of mammals have unique characteristics. The cerebral cortex, the outermost part of the cerebrum, is highly folded, allowing for a greater surface area than if it were smooth. The optic lobes in the mid-brain are divided into two parts, whereas other vertebrates have an undivided lobe. Most mammals have a special structure called the corpus callosum that links the two cerebral hemispheres together and functions to integrate motor, sensory, and cognitive functions between the left and right cerebral cortexes.
The major groups of mammals living today are the monotremes (prototheria), marsupials (metatheria), and placental (eutheria) mammals. There are few species of monotremes, which are the platypus and four species of echidnas, which are spiny anteaters. The platypus with its leathery beak is differentiated from the echidna and its sticky tongue. The platypus and one species of echidna are found in Australia, while the other species of echidnas are found in New Guinea. Monotremes are unique among mammals because they lay eggs instead of giving live births. These egg shells are leathery and not hard like bird egg shells but more like reptile egg shells. The eggs are laid in nests and a yolk-sac placenta helps support development. The babies hatch in a fetal state and continue to develop in the nest, and nourished by milk secreted by mammary glands opening directly to the skin. Adult monotremes do not have teeth. The average temperature of monotreme bodies is 30 degrees Celsius, which is considerably lower than other mammals that have body temperatures 35-38 degrees Celsius.
Over two thirds of marsupials are found in Austrailia, New Guinea, and nearby islands. The opossum, a marsupial, is found in the Americas. Australian marsupials include the kangaroo, koala, bandicoot, Tasmanian Devil, and a few others. The embryos of marsupials are nourished during a short gestational period by a yolk-sac placenta, but no egg shell. Some marsupial embryos can suspend development until implantation is complete. Marsupials are usually fetal at birth and most marsupials possess a pouch where the young marsupials live, receive milk, and continue development. Young kangaroos are nursed for a year and a half.
Eutherians or placental mammals are the most common mammals around the world. The distinguishing feature is the placenta that connects the mother to the developing fetus and allows the exchange of gas, fluid and nutrients. There are over 4,000 species of placental mammals and many of these have unique abilities of burrowing, flying, swimming, hunting, running, and climbing. Placental mammals include elephants, hyraxes, manatees, anteaters, armadillos, sloths, tree shrews, rodents, squirrels, porcupines, rabbits, hares, horses, rhinos, cows, giraffes, pigs, hippos, whales, cats, dogs, bears, bats, flying foxes, primates, and humans.
Primates are a class of mammals that include lemurs, tarsiers, monkeys, and apes. Non-human primates live in tropical and sub-tropical regions and their sizes range from the mouse lemur to the mountain gorilla. Primates are unique in ability to climb trees and live in trees. Primate hands and feet are designed for climbing trees and swinging through trees using their arms. These abilities are made possible by the rotating shoulder joint, a big toe that is widely separated from other toes, thumbs widely separated from other fingers allow greater grasping ability, and stereoscopic vision allows overlapping fields of vision to see depth and distance. Primate brains are larger than most other mammals, their claws have flattened nails, primates usually have one offspring per pregnancy, and primates hold their body upright. Primates have been classified into groups of "turned-nosed," and "simple-nosed." Some primates are classified as wet-nosed or dry-nosed. Turned-nosed primates are commonly nocturnal
The design of vertebrate animals includes animals that have a backbone and a skeleton by definition.
Phylum Chordata (Chordates) have five key characteristics: a notochord, a dorsal hollow (tubular) nerve chord, pharyngeal gill arches or slits, a post-anal tail, and an endostyle /thyroid gland.
The notochord, which chordates are named, is a flexible, rod-shaped mesodermal structure that is found in the embryonic stage of all chordates and some of the adult chordates. The notochord provides rigidity, skeletal support, and flexibility. The notochord is located between the digestive tube and the nerve chord.
The dorsal hollow nerve cord rolls into a hollow tube during development and is located dorsally from the notochord.
Pharyngeal slits are openings in the pharynx, or the region just posterior to the mouth that extend to the outside environment. These slits allow for the exit of water in aquatic environments that enters from the mouth. Some chordates use the slits for filtering food from the water.
The post and tail is a posterior elongation of the body, extending beyond the anus. The tail contains skeletal elements and muscles, that allow for locomotion in aquatic environments and also for terrestrial varieties helps with balance, courting, and signaling.
Two types of chordates are invertebrates: Cephalochordata and Urochordata
Cephalochordata members have a notochord, dorsal hollow tubular nerve chord, pharyngeal slits, endostyle/thyroid gland, and post-anal tail in the adult stage. Lancets are a few centimeters long and are found buried in the sand at the bottom of warm, tropical seas. Cephalochordates are suspension feeders. Water current is created by cilia in the mouth and the water is filtered through oral tentacles. The pharyngeal slits filter out food particles. The filtered water collects in a gill chamber called the atrium and exits through the atriopore.
Urochordata (or tunicates) have a cellulose-like carbohydrate material called tunic which covers the outer body. Adult tunicates do not have a notochord, dorsal hollow nerve cord, or a post-anal tail, but do have pharyngeal slits and an endostyle. The larval form contains all five of these structures, but three disappear upon adult form. Most tunicates are hermaphrodites. Adult tunicates can be either solitary or colonial. Most tunicates are suspension feeders.
Subphylum Vertebrata (Craniata) features the cranium, a bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and face bones. These are bilaterally symmetrical animals that include the jawless hagfishes, lampreys, and all vertebrates. Hagfish have a cranium but lack a backbone. Members of Craniata/Vertebrata have the five characteristic features of chordates. However, all vertebrates have a vertebral column, composed of vertebrae, which are a series of irregularly shaped bones joined together to form a backbone. The vertebrates are the largest group of chordates.
Jawless fishes are known as the superclass Agnatha. Class Myxini contains the hagfishes, lenticular scavengers that live on the bottom of the ocean floor and eat any animals available. Hagfish have poor eyesight, but have sensory organs on their mouth and have teeth that can pull flesh from prey. The gills are used for respiration and their body surfaces contain slippery mucus substances produced inside their skin that allow them to evade predators. Their bodies are also flexible and they can tie their bodies into a knot. Hagfishes have a cartilaginous skull and a fibrous and cartilaginous skeleton, while the notochord runs the length of the body and provides support.
Class Petromyzontida includes the lampreys, which are similar to hagfish in size and shape. However, lampreys have extrinsic eye muscles, two or more semi-circular canals, a cerebellum, and cartilaginous vertebrae above the notochord. Lampreys have a dorsal tubular nerve cord with a well differentiated brain, a small cerebellum, and 10 pairs of nerves. Lampreys have one or two fleshy dorsal fins, and teeth around its mouth. Many lampreys have a parasitic stage in their life. Lampreys live in coastal and freshwater environments around the world in temperate climates. Lamprey eggs fertilize externally.
Gnathostomes, or jaw-mouths, are vertebrates with true jaws: hinged structures attached to the cranium that allow for grasping and tearing food. However, evolutionists cannot explain how these animals "suddenly" obtained jaws in the fossil record. In addition to jaws, gnathostomes have paired appendages, teeth, specialized embryonic and skeletal tissues.
Class Condrichthyes are the cartilaginous fishes such as sharks, rays, skates, and sawfishes. Condrichthyes are jawed fishes with paired fins and a skeleton made of cartilage. They are mostly marine with a few freshwater varieties and are carnivorous. Shark skin has tooth-like scales called placoid scales. Some sharks and skates are suspension feeders. The sawfish front looks like a double-edged saw and the rostrum, or pointed snout, is covered with electrosensitive pores that help the animal find prey. Electroreceptors called ampullae of Lorenzini allow sharks to detect electromagnetic fields that are produced by all living things, including their prey. Sharks and most fishes and amphibians have a row of sensory structures called the lateral line, which is used to detect movement and vibration in the surrounding water and is functionally similar to hearing in land vertebrates. The lateral line is visible as a dark stripe along the length of the body. Sharks must swim continuously to maintain buoyancy with the water.
Sharks reproduce sexually and eggs are fertilized internally in the female. In ovoviviparous sharks, the eggs hatch in the uterus and the newborn hatched sharks are alive and fully functional. Some sharks, however, are oviparous, that lay eggs externally from the body that are protected inside a pouch case that has tentacles that grab seaweed to protect the eggs. Some sharks are viviparous, or give live birth after fertilization and embryo development occur inside the mother's body.
Chondrichthyes usually have a flattened body, unequal sized fin lobes, with the tail vertebrae extending into the larger upper lobe, paired pectoral and pelvic fins, exposed gill slits, and intestine with a spiral valve that condenses the length. They also have three pairs of semi-circular canals, excellent senses of smell, vibration, vision, and electroreception.
Rays and skates are similar to sharks but their bodies are flatter, their pectoral fins are enlarged and fused to their head, and their gill slits are located on their ventral surface. Rays and skates have cartilaginous skeletons just like sharks and most are marine and live on the seafloor.
Holocephali (chimaeras or ratfish) have a diphycercal tail with equal sized fin lobes, and tail vertebrae located between them. Holocephali lack scales and have teeth like grinding plates used to feed on mollusks and other invertebrates. Chimaeras have four pairs of gills covered by an operculum.
Osteichthyes are the bony fishes and are characterized by a bony skeleton. Most bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that produce and maintain a calcium phosphate matrix. However, sturgeon and paddlefish have mostly cartilaginous skeletons. The skin of bony fishes usually contains overlapping scales and skin mucus aids in swimming dynamics and osmoregulation. All bony fish use gills to breathe, as water is drawn over the gills that are located in chambers covered and ventilated by a protective flap called an operculum. Many bony fishes also have a swim bladder filled with gas as a pouch from the gut to control buoyancy and gases of the swim bladder are exchanged with the blood.
Two classes of bony fishes are ray-finned (Actinopterygii) and lobe-finned (Sarcopterygii). Ray-finned fishes include tuna, bass, trout, salmon and have a fan of slender bones that supports their fins. The lobe-finned fishes are fleshy and lobed, supported by bones, and include the lungfish and coelacanths, which are living fossils once thought to be extinct.
Amphibians are vertebrate tetrapods with four limbs, including frogs, salamanders, and caecilians. Most amphibians undergo a life cycle from water to land. Most amphibians have moist, permeable skin made possible by mucus glands. Amphibians intake more water from their skin than by drinking. Amphibians also breathe through their skin, in addition to their lungs and mouth. Amphibians are carnivorous, aided by a sticky tongue used to catch prey, along with teeth and jaws. Amphibians have teeth on both sides of the jaw or on one side of the jaw. Some amphibians have multiple rows of jaws. Some amphibians have vomerine in the roof of the mouth. Amphibians have eyes for vision and can see color. Ears allow amphibians, particularly frogs and toads, to communicate with each other by vocalization. Different sections of the inner ear can detect different frequencies of sounds below and above 10,000 hertz. The papilla amphibiorum can detect below 10,000 hertz while the papilla basilaris can detect above 10,000 hertz such as mating calls. The operculum in the ear can detect seismic signals. Amphibian classes are divided into Urodela (tailed), Anura (tail-less), and Apoda (legless).
Salamanders (Urodela) include both aquatic and terrestrial types with four limbs and a tail. Most salamanders do not have lungs and breath through their skin or external gills. Some land salamanders have lungs. Some salamanders have lungs and gills. Salamanders reproduce using internal fertilization of the eggs. Sperm of the males is deposited in a packet called a spermatophore and picked up by the female. Eggs are hatched after being laid in most salamanders (oviparous). Aquatic salamanders lay their eggs in water, and the eggs hatch into larvae. Land salamanders lay their eggs in nests, and are guarded by the mother. These larvae gain adult form before hatching. The Mexican axolotl stay in the larvae stage.
Frogs (Anura) are found on all continents except Antarctica. Frogs vary greatly in size from millimeters to centimeters. Their hind legs are capable of jumping in order to catch prey. Tree frogs have hands specialized for grabbing branches as they climb. Flying frogs in tropical environments can move with their webbed feet. Their skin acts as camouflage. Frogs and salamanders have chemical defense systems in their skin glands. Frog eggs are fertilized externally in moist environments. Some frogs do not perform parental care of eggs, while others carry their eggs on their body or place them on a leaf. The life cycle of most frogs involves the larval stage and the metamorphosis into adult stage. However, some frogs develop directly from eggs to adult stage. The larval frog, or tadpole, is a filter-feeding herbivore. Tadpoles have gills, a lateral line system, longfinned tails, and no limbs. After metamorphosis, gills, tail, and lateral line disappear and four limbs develop. The rest of the body becomes larger and more mature in adult form and allow the adult to move onto land.
Apoda (Caecilians) are a group of amphibians that have no limbs and resemble earthworms. There are also folds of skin that resemble the segments on earthworms. However, unlike earthworms, caecilians have teeth in both jaws, and eat organisms in the soil, including earthworms. Caecilians can burrow in the soil or swim, but have poor eyesight. Caecilians have a single lung and breathe through their skin. Caecilians are found in tropical environments of the world of Africa, Asia, and South America. Males have copulatory structures to transfer sperm into the female, and fertilization is internal. Some caecilians are oviparous (eggs hatch outside the mother's body), but most caecilians produce live births.
Reptiles
The features of reptiles include land-based eggs, which has four extraembryonic membranes: the yolk sac, the amnion, the chorion, and the allantois. The chorion and amnion develop from folds in the body wall, and the yolk sac and the allantois are extensions of the midgut and hindgut respectively. The amnion forms a fluid-filled cavity that provides the embryo with its own aquatic environment. The eggs of reptiles have shells. An amniote embryo was then enclosed in the amnion, which was then enclosed in an extra-embryonic coelom contained within the chorion.
An amniote is an animal whose embryo develops in an amnion and chorion and has an allantois. Amniotes include mammals, birds, or reptiles. The amniotic egg is the key characteristic of amniotes. The shell of the egg provides protection while allowing exchange of carbon dioxide and oxygen gas. The albumin, or egg white, outside the chorion provides the embryo with water and protein, while the egg yolk provides nutrients for the embryo. Blood vessels in the yolk sac transport yolk nutrients to the circulatory system of the embryo. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg's external environment. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. The amnion protects the embryo from mechanical shock and supports hydration.
Some reptiles are tetrapods, having four legs, but some reptiles, like snakes, do not have any legs. Reptile eggs are calcareous or leathery and all reptiles lay eggs on land, even aquatic reptiles return to land to lay eggs. Reptiles usually reproduce sexually with internal fertilization. In some reptile species, eggs stay in the mother until they are ready to hatch. In other species, babies are born alive.
Many reptiles have scaly skin containing the protein keratin and waxy lipids, which help reduce water loss from the skin. Some reptiles have claws or nails or hair. Reptiles cannot use their skin for respiration, and instead use lungs. Reptiles grow throughout their lives and shed their skin regularly as it helps them grow and to remove parasites. Reptiles use their muscles to ventilate their lungs and inhale air by reducing pressure. Most reptiles are ectotherms, and their body heat comes from the environment, and their bodies don not produce heat themselves. Therefore, reptiles can go long periods without food because they don't need as much energy from food to heat their body. The reptile can survive even longer without food by being inactive and conserving energy during colder weather, called brumation.
Crocodilia is a type of reptile that includes the alligators and crocodiles that live in tropical and subtropical climates. They can live in freshwater, saltwater, rivers, lakes, and stay in the water most of the time. Their legs help them swim in the water and also walk on land. Some species of crocodiles can even gallop.
Spenodontia are known commonly as tuatara named for the crest on its back and has a skull and vertebrae. It has four legs and a tail. Length is up to 80 centimeters. Tuataras have no external ears. They have a third eye in between the two main eyes which is not visible in adults. Their jaws have two rows of teeth in the upper jaw and a single row on the bottom jaw. Tuataras are found in New Zealand.
Squamata are scaly lizards and snakes found on all continents except Antarctica. Lizards differ from snakes in having four legs to none. Snakes do not have eyelids and external ears, while lizards do. Lizards include the small chameleons and geckos, and the large Komodo dragon. Lizards usually have spines, crests, and frills, and many are brightly colored. Chameleons can change their skin color as pigment is redistributed in the chromatophore cells in the skin. They change color for camouflage to help avoid predators and for social signaling. Lizards have color vision and can focus their eyes by changing the shape of the lens. The eyes of chameleons can move independently. Some lizards have a parietal third eye like the tuatara. Both lizards and snakes use their tongue to sample the environment and Jacobson's organ in the roof of the mouth helps identify a sample. Most lizards are carnivorous, but some are herbivores. Lizard tongues are long and sticky and enabling the catch of prey. Some lizards have venom for self defense. Special features of the jaw help ingestion of large prey in reptiles and all snakes are carnivorous.
Snakes range from 10 centimeters to 10 meters in length. All snakes are legless and have a single lung. Most snakes have a flexible and rotational skull. The lower jaw does not have a bone or ligament connection, enabling a great expansion for eating larger animals. Most snakes are non-venomous and swallow their prey whole. Venom helps capture prey in those that have it and in digestion. Snakes have no eyelids, but do have a protective transparent scale. Some snakes can see ultraviolet light, which helps in finding prey. Snakes have inner ears and some snakes can detect infrared heat emissions from prey with a sensory organ between the eyes and nostrils. Snakes also use Jacobson's organ for sensing.
Testudines include the turtles, terrapins, and tortoises, which are characterized by a bony or cartilaginous shell. The shell in turtles is not just an outer covering, but is built into the skeletal system. The dorsal shell is called the carapace and includes the backbone and ribs, while the ventral shell is called the plastron. Both shells are covered with keratinous plates or scutes, and the two shells are held together by a bridge. Some turtles can hide their head and legs under the shell by way of a hinge in the shell. In the two groups of living turtles, the Cryptodira retracts its neck in a vertical S-curve. Pleuroida retracts their necks with a horizontal curve, folding the neck to the side. Turtles are ectotherms like most reptiles and depend on heat from the environment. All turtles lay their eggs on land, but many turtles live in or near water. Parental care is non-existent. Turtle size can range from 8 centimeters to 200 centimeters.
Birds have common features including feathers and wings. Birds are endothermic so that they produce their own body heat and maintain a constant internal temperature so that they can remain active daily and yearly throughout the world, have faster nerve impulses, increased muscle strength, and greater physical endurance. Birds are homeothermic in that they can maintain an elevated and constant body temperature. Bird temperature is higher than most mammals because birds produce much energy from their active lifestyle. Bird feathers also help retain internal body heat.
Birds have two main types of feathers: contour feathers and down feathers. Contour feathers have a number of parallel barbs that branch from a central shaft. The barbs have microscopic branches called barbules linked together by small hooks and this structure helps the feather to be strong, flexible, and have a complete, uninterrupted surface. The barbules of down feathers do not interlock, making these feathers good for insulation by trapping air between the spaces of the barbules of adjacent feathers, reducing heat loss.
Feathers also provide the necessary lift for flight while the flexibility of the feathers reduces drag during flight on the wings. The asymmetrical curved shape of the wing allows lift to occur during flight. Flight feathers are divided into primary and secondary feathers. Primary feathers are located at the tip of the wing and provide thrust as the bird moves its wings downward, using major muscles. Secondary feathers are located closer to the body in the forearm of the wing, and provide lift. Contour feathers on the body help reduce drag produced by wind resistance during flight, creating a smooth surface for air to move over while preventing turbulence.
Flapping of the wing is made possible by chest muscles, including the pectoralis major muscles that move the wings downward, while the supracoracoideus muscles move the wings upward. The flight muscles are rich in oxygen-storing myoglobin. The collarbones are attached, forming the furcula or wishbone, which is flexible but supportive of flight.
Flying birds must have a low body weight and the heaviest birds are usually flightless, like the ostrich. Pneumatic bones in birds are hollow, and not filled with tissue, but have "cross struts" in between the bones called trabeculae for structural support. Not all bones in the bird skeleton are pneumatic, but many are including the skull. Beaks are made of keratin, a lighter material that helps reduce weight. The lack of a urinary bladder also reduces weight. Birds have a cloaca, an external body cavity where the intestinal, urinary, and reproductive organs empty and water can be recycled into the blood. Uric acid is converted to solid form and released with other solid waste so that the bird does not need to store extra liquid urine in a bladder, and helping to reduce weight. Female birds also commonly have only one ovary instead of two as in other animals, and therefore weight is reduced.
The respiratory system of birds allows for the high metabolic rate required for flight. Air spaces in the pneumatic bone are sometimes connected to air sacs in the body cavity, which replace coelomic fluid and lighten the body. These air sacs are also connected to the path of air flow through the birds body, and function in respiration. Airflow in bird lungs is unidirectional, unlike in two directions in mammals. Gas exchange occurs in air capillaries or microscopic air passages within the lungs. The arrangement of air capillaries in the lungs creates a cross-current exchange system with the pulmonary blood. In a counter-current system, the air flows in one direction and the blood flows in the opposite direction, producing a favorable diffusion gradient and creating an efficient means of gas exchange. This effective oxygen-delivery system supports higher metabolic activity. Ventilation is provided by the parabronchi expandable lungs within the air sacs located among the visceral organs and skeleton. A syrinx voice box resides near the junction of the trachea and bronchi.
Other unique feature of birds are the elongated, s-shaped neck, and short tail connected to the vertebrae. Bird vertebrae structure allows for more flexibility than mammal necks. Birds have a thin epidermis and no sweat glands. A specialized uropygial gland, or sebaceous "preening gland" found at the dorsal base of the tail. This gland is essential to continuous preening as an oily substance is produced that helps birds waterproof their feathers and keep their feathers flexible. Some birds do not have this gland, but have special feathers that produce a powdery down that has a similar effect as the gland.
Birds have 12 pairs of cranial nerves like mammals and a large cerebellum with optic lobes. Birds also have a single bone in the middle ear called the columella. Birds have a closed circulatory system with two atria and two ventricles with a right-bending aortic arch (left in mammals), along with nucleated red blood cells (enucleated in mammals). Birds are unique in the animal world with their large brains, strong senses, vocalization ability, and tool use.
Mammals are vertebrate animals that have hair and mammary glands. Most mammals are placental, except monotremes and marsupials. Certain other features of anatomy are characteristic of mammals. Hair in mammals is composed of the protein keratin and this hair has many important functions. Since mammals are endothermic and produce their own heat, hair on mammals helps retain heat generated by metabolic activity as insulation. Hair also serves as a sensory mechanism by specialized hairs called vibrissae or whiskers, which are attached to nerves that transmit information about tactile vibration produced by sound sensation. Hair can also serve as protective coloration or social signaling, when an animals hair stands up or outward to scare predators.
The skin of mammals contains several types of secretory glands not found in birds. Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin, providing water resistance and lubrication for hair and these are located on most of the body. Eccrine glands produce sweat, or perspiration, which is mainly composed of water, but also contains metabolic waste products, and sometimes compounds with antibiotic activity. In most mammals, eccrine glands are limited to certain areas of the body, and some mammals do not have them at all. In primates and humans, sweat glands are located across most of the body surface and these help regulate body temperature. Apocrine glands, or scent glands, secrete substances that are used for chemical communication, as do skunks. Mammary glands produce milk that is used to feed newborn babies. Evolutionists have difficulty explaining the origin of mammary glands.
Mammal skeletal systems feature a lower jaw with only one bone, the dentary, and the jaw hinge connects the dentary to the flat part of the temporal bone in the skull. Other vertebrate jaws have several bones. In mammals, the quadrate and articular bones are in the middle ear. The stapes, incus, and malleus are bones of mammal ears going from inward to outward.
The adductor muscles that close the jaw are the two major muscles in mammals: the temporalis and the masseter. These muscles allow up and down and side to side movements of the jaw, so that chewing is possible, which is a unique characteristic of mammals. Most mammals have heterodont teeth meaning different types and sizes of teeth, such as incisors, canines, and molars. Most mammals are also diphyodonts meaning that they have two sets of teeth in their life: baby teeth and permanent teeth. A few mammals and other vertebrate animals replace teeth throughout their lives.
Mammals are like birds in that they have a four-chambered heart. Mammals also have a specialized group of cardiac cells or fibers located in the walls of their right atrium called the sinoatrial node, or pacemaker, which determines the rate at which the heart beats. Red blood cells in mammals do not have nuclei, but other vertebrates have nucleated red blood cells.
The kidneys of mammals have a nephritic loop which allows mammals to produce urine with high concentrates of solutes, even higher than blood. A urinary bladder is present in all mammals.
The skulls of mammals have two occipital condyle bones at the base of the skull that associate with the first vertebrae and a secondary palate at the rear of the pharynx that separates the pathways of swallowing and breathing. Turbinate bones or conchae are located along the sides of the nasal cavity that help warm and moisten air as it is inhaled. The pelvic bones are fused in mammals, and there are usually seven cervical vertebrae. Mammals have moveable eyelids and fleshy external ears called pinnae. Mammals have a muscular diaphragm.
Brains of mammals have unique characteristics. The cerebral cortex, the outermost part of the cerebrum, is highly folded, allowing for a greater surface area than if it were smooth. The optic lobes in the mid-brain are divided into two parts, whereas other vertebrates have an undivided lobe. Most mammals have a special structure called the corpus callosum that links the two cerebral hemispheres together and functions to integrate motor, sensory, and cognitive functions between the left and right cerebral cortexes.
The major groups of mammals living today are the monotremes (prototheria), marsupials (metatheria), and placental (eutheria) mammals. There are few species of monotremes, which are the platypus and four species of echidnas, which are spiny anteaters. The platypus with its leathery beak is differentiated from the echidna and its sticky tongue. The platypus and one species of echidna are found in Australia, while the other species of echidnas are found in New Guinea. Monotremes are unique among mammals because they lay eggs instead of giving live births. These egg shells are leathery and not hard like bird egg shells but more like reptile egg shells. The eggs are laid in nests and a yolk-sac placenta helps support development. The babies hatch in a fetal state and continue to develop in the nest, and nourished by milk secreted by mammary glands opening directly to the skin. Adult monotremes do not have teeth. The average temperature of monotreme bodies is 30 degrees Celsius, which is considerably lower than other mammals that have body temperatures 35-38 degrees Celsius.
Over two thirds of marsupials are found in Austrailia, New Guinea, and nearby islands. The opossum, a marsupial, is found in the Americas. Australian marsupials include the kangaroo, koala, bandicoot, Tasmanian Devil, and a few others. The embryos of marsupials are nourished during a short gestational period by a yolk-sac placenta, but no egg shell. Some marsupial embryos can suspend development until implantation is complete. Marsupials are usually fetal at birth and most marsupials possess a pouch where the young marsupials live, receive milk, and continue development. Young kangaroos are nursed for a year and a half.
Eutherians or placental mammals are the most common mammals around the world. The distinguishing feature is the placenta that connects the mother to the developing fetus and allows the exchange of gas, fluid and nutrients. There are over 4,000 species of placental mammals and many of these have unique abilities of burrowing, flying, swimming, hunting, running, and climbing. Placental mammals include elephants, hyraxes, manatees, anteaters, armadillos, sloths, tree shrews, rodents, squirrels, porcupines, rabbits, hares, horses, rhinos, cows, giraffes, pigs, hippos, whales, cats, dogs, bears, bats, flying foxes, primates, and humans.
Primates are a class of mammals that include lemurs, tarsiers, monkeys, and apes. Non-human primates live in tropical and sub-tropical regions and their sizes range from the mouse lemur to the mountain gorilla. Primates are unique in ability to climb trees and live in trees. Primate hands and feet are designed for climbing trees and swinging through trees using their arms. These abilities are made possible by the rotating shoulder joint, a big toe that is widely separated from other toes, thumbs widely separated from other fingers allow greater grasping ability, and stereoscopic vision allows overlapping fields of vision to see depth and distance. Primate brains are larger than most other mammals, their claws have flattened nails, primates usually have one offspring per pregnancy, and primates hold their body upright. Primates have been classified into groups of "turned-nosed," and "simple-nosed." Some primates are classified as wet-nosed or dry-nosed. Turned-nosed primates are commonly nocturnal