Phylum Chordata

Characteristics of Chordates:

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.

Formation of Nerve Cord

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.

Taxonomy of Chordates:

The main taxa within Phylum Chordata include:

labeled Amphioxus l.s.
Labeled Microscopic View of Amphioxus

Subphylum Cephalochordata (cephalo = head), which doesn’t have a “head.” This subphylum includes lancelets (or 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; saur = lizard), snakes, turtles, crocodiles, and lizards. Reptiles have scales and are dry to the touch. Their eggs have leathery shells. Reptiles are exothermic (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. (It is also now known that many kinds of dinosaurs had 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.
- The subclasses and orders within Class Mammalia include:
  - Subclass Monotremes (trema = hole) includes the platypus and spiny anteater. 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.
  - Subclass Marsupials (marsupi = a bag, pouch) includes opossums, kangaroos, koalas, etc. 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.
    - Order Marsupialia
  - 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:
    - Order Artiodactyla includes sheep, pigs, cattle, deer, giraffes, and goats which have even-toed hooves and are herbivorous.
    - Order Carnivora (carni = flesh; vore = eat, devour) includes cats, dogs, bears, seals, walruses, skunks, and racoons. These are carnivorous and have pointed canine teeth and molars.
    - Order Cetacea (ceta = a whale) includes whales, dolphins, and porpoises which are aquatic/marine. 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.
    - Order Chiroptera (chiro = a hand, like chiropractor; ptera = wing, feather) is the bats. Their fingers are webbed to create wings.
    - Order Edentata includes sloths, anteaters, and armadillos. They have reduced or no teeth.
    - Order Insectivora (vora = to eat, devour) includes moles, shrews, and hedgehogs which eat insects.
    - Order Lagomorpha includes the rabbits, which have chisel-like incisors and hind legs modified for jumping.
    - Order Perissodactyla includes horses, tapirs, and rhinoceros which have odd-toed hooves and are herbivorous.
    - Order Primates (prima = first) includes lemurs, monkeys, apes, and humans. These have opposable thumbs and eyes that face forward.
    - Order Proboscidae includes elephants which have long, muscular trunks and thick, loose skin. Their upper incisors are elongated as tusks.
    - Order Rodentia (roden = gnaw, gnawing) includes squirrels, beavers, rats, porcupines, woodchucks, guinea pigs, and mice. These have chisel-like, constantly-growing incisors.
    - Order Sirenia includes manatees which are aquatic herbivores. Their front legs (“arms”) are fin-like, and they have no hind legs.

Note that humans (Class Mammalia) are to reptiles (Class Reptilia) as insects (Class Insecta) are to centipedes (Class Chilopoda), and humans (Subphylum Vertebrata) are to Amphioxus (Subphylum Cephalochordata) as insects (Subphylum Atelocerata) are to spiders (Subphylum Chelicerata), or the other way around, insects and spiders are no more closely related to each other than humans and lancelets.

References:

Borror, Donald J. 1960. Dictionary of Root Words and Combining Forms. Mayfield Publ. Co.

Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology, 5^th Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)

Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology: Concepts and Connections, 3^rd Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)

Marchuk, William N. 1992. A Life Science Lexicon. Wm. C. Brown Publishers, Dubuque, IA.

Zimmer, Carl. 2011. The long curious extravagant evolution of feathers. Natl. Geog. 219(2):32-57.

times since 02 Jan 2002.