All chordates have a number of structures in common:
A notochord (noto = the back; chord = string) is present in all embryos, and may be present or absent/reduced in adults. This is the structure for which the phylum was named.
A dorsal, hollow, ectodermal nerve cord (compare with Annelida and Arthropoda which have ventral, solid, mesodermal nerve cords) typically forms by an infolding of the ectoderm tissue, which then “pinches off” and becomes surrounded by mesoderm. Spinal bifida is the failure of the nervous system to close.The pharangeal slits (pharynx = throat) originally functioned in filter feeding: water is taken into the mouth and let out via the pharangeal slits. The slits filter out food particles and keep them in the animal’s body so they can be put into the digestive tract. In fish, these have become modified as gills, and in humans as our ears and eustachian tubes.
A postanal tail (post = behind, after; anal refers to the anus) is present and extends behind the anus in many taxa, thus the anus isn’t at posterior tip of body. In humans, the tail is present during embryonic development, but is subsequently resorbed.
The main taxa within Phylum Chordata include:
Subphylum Cephalochordata (cephalo = head), which doesn’t have a “head.” This subphylum includes Lancets, so-named for their shape. These are in genus Branchiostoma (branchio = gill, fish; stoma = mouth) which was formerly know as genus Amphioxus (amphi = on both sides, double) Lancets look a lot like the generic chordate described above. They have a notochord, but no bones. They are about one inch long, and live in the muddy ocean floor.
Subphylum Urochordata (uro = tail) are called the tunicates. Their larvae show typical chordate characteristics, but the adults have lost many of these organs, and what they do still have have become highly-modified.
Subphylum Vertebrata may have come from an Amphioxus-like ancestor, however vertebrates have a definite head and Amphioxus doesn’t. Another theory suggests that vertebrates may have come from a larval form similar to tunicate larvae that were capable of reproduction in the larval stage, that is, they are sexually mature prior to metamorphosis, which is referred to as paedogenesis (paedo = child — same root as in pediatrician; genesis = origin, birth), defined as the precocious attainment of sexual maturity in a larva. Vertebrate characteristics include:
vertebrae, the skeletal units surrounding the nerve cord
a brain, enclosed within a skull
an endoskeleton which will grow along with the animal (unlike arthropods which must molt).
a closed circulatory system with a ventral heart
excretion via kidneys
separate males and females with sexual reproduction in most, with a few cases of parthenogenesis
The Classes in Subphylum Vertebrata include:
Class Agnatha (a- = not, without; gnatho = jaw) which is the lampreys. They do not have jaws, are eel-shaped, prey on fish, and have larval forms which are different from the adults. |
Class Chondrichthyes (chondro = cartilage; ichthys = fish) which includes sharks and rays. They have a cartilage skeleton, not bone. They are not buoyant like other fish so they must swim or sink. Like other fish they have a lateral line system which detects differences in water pressure, the equivalent of our hearing.
Class Osteichthyes (osteo = bone) is the bony fish. This is the most numerous of all vertebrate classes. In fish, O2 is exchanged via the gills, which are covered by an operculum which helps to draw water across/through the gills. Their swim bladder is an air sac used to control buoyancy, thus unlike the sharks, bony fish can hold still at any depth and not sink.
Class Amphibia (amphi = on both sides, double; bios = life) is frogs, newts, and salamanders. They were the first land vertebrates. Frogs, especially, go through metamorphosis. Their eggs have no egg shells, so the sperm can swim through the water to the eggs, and the embryos must develop in water.
Class Reptilia (reptili = creeping) is the dinosaurs (dino = terrible ), snakes, turtles, crocodiles, and lizards. Reptiles have scales and are dry to the touch. Their eggs have leathery shells. Reptiles areexothermic (exo = out, outside), that is they maintain their body temperature through external means such as sunning on a rock or seeking shade. Reptiles need less food/energy to live and live longer than a comparable-sized mammal. Some dinosaurs may have been endothermic.
Class Aves (avi = a bird) is the birds. It is thought that birds are descended from dinosaurs, as evidenced, in part, by the scales on their feet. Also, feathers are modified scales: a key characteristic of birds is that they have feathers. Birds’ bones are light weight for flight. Birds are endothermic (endo = within, inner), that is, they control their body temperature from within (they’re “warm-blooded”). Birds’ vision is the best of all vertebrates: soaring hawks can spot small mice scrambling through the grass in a field far below them. Birds have shelled eggs and so must have internal fertilization — the egg muct be fertilized before the hen’s reproductive tract secretes an eggshell. Generally, mating is accompanied by an elaborate courtship ritual. Eggs and often young birds are more exothermic (are not able to control their body temperatures from within) and so must be brooded/incubated by parents.
Class Mammalia (mamma, mammil = teat, nipple) is the mammals. Key characteristics of mammals are the presence of fur/hair and mammary glands, derived from modified sweat glands, which produce milk for the young. Mammals have a diaphragm to aid in respiration. They are endothermic. Most mammals bear live young.
Animal Groups | Description (Which group do you like best?) | Details |
 | Playtypuses, echidnas. Generally regarded as the most primitive mammals since they lay eggs (like reptiles) and do not give birth to formed young. These mammals lays eggs like reptiles, but do have fur and milk. However, they have no nipples: their mammary glands just secrete milk onto the fur, from which the babies lick it. They have one posterior opening for their digestive, urinary, and genital tracts, hence the subclass name. | |
Includes kangaroos, wallabies, wombats, opossums, dunnarts, bandicoots, cuscuses. The young are born as very immature embryos and must crawl to their mother’s pouch to continue their development. Typically, once in the pouch (marsupium) a baby would find a nipple there. Because of continental drift, there is a wide variety of marsupials in Australia, yet few elsewhere on Earth. | ||
Subclass Placentals (placent = a round, flat cake) contains most of the animals with which we are familiar. In this taxon, young complete embryonic development within the mother’s uterus and are nourished across a placenta. Some of the Orders in Subclass Placentals include: | ||
 | Some of the worlds most bizarre animals - anteaters, armadillos, sloths. They have reduced or no teeth. | |
Includes lemrus, bushbabies, lorises, pottos, tarsiers, marmosets, tamarins, monkeys, apes, humansOrder Primates (prima = first) includes lemurs, monkeys, apes, and humans. These have opposable thumbs and eyes that face forward. | ||
 | Seven living species of mammal are included in the Pholidota, the pangolins or scaly anteaters. | |
 | Includes shrews, moles, golden moles, desmans, hedgehogs, moonrats, solenodons, tenrecsOrder Insectivora (vora = to eat, devour) includes moles, shrews, and hedgehogs which eat insects. | |
 | Aardvark | |
 | Includes big and small cats, dogs, foxes, wolves, hyenas, bears, pandas, raccoons,weasels, stoats, badgers, skunks, otters,mongooses, civets. These are carnivorous and have pointed canine teeth and molars. | |
 | Seals, sea lions, walruses | |
 | Also called flying lemurs. | |
 | Includes flying foxes (fruit bats), vampires, and all other bats. (chiro = a hand, like chiropractor; ptera = wing, feather) is the bats. Their fingers are webbed to create wings. | |
 | Includesrats and mice, dormice, gerbils, beavers, squirrels, porcupines, chinchillas, pacas, voles, hamsters,chipmunks. Order Rodentia (roden = gnaw, gnawing) includes squirrels, beavers, rats, porcupines, woodchucks, guinea pigs, and mice. These have chisel-like, constantly-growing incisors | |
 | includes rabbits, cottontails, jackrabbits, hares, pikas. which have chisel-like incisors and hind legs modified for jumping. | |
 | toothed whales, baleen, (whalebone) whales, dolphins, propoises. Their front legs (equivalent to our arms) are fin-like, and they have no hind legs. They have a thick layer of blubber for insulation. | |
 | Includes hippos, pigs, peccaries, camels, llamas, giraffes, deer, chevrotains, gazelles, antelopes, cattle, sheep, goats. which have even-toed hooves and are herbivorous. | |
 | horses, asses, zebras, rhinos, tapirs. which have odd-toed hooves and are herbivorous. | |
 | bush hyraxes, tree hyraxes, rock hyraxes, dassies. | |
 | African and Indian elephantswhich have long, muscular trunks and thick, loose skin. Their upper incisors are elongated as tusks. | |
 | Dugongs, manatees. Their front legs (“arms”) are fin-like, and they have no hind legs. | |
 | tree shrews, tupais, and dendrogales or tree squirrels |
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