Primitive Chordates

Tunicates (sea squirts): barrel-shaped sessile (not moving) marine organisms similar in appearance to sponges or cnidarians like hydra. Sometimes found in colonies.

Lancets: small, fish-shaped animals which are segmented, but have no sense organs, heart, or well-defined brain.

Jawless Fish

Lampreys. Usually freshwater, although some spend adulthood in the oceans after undergoing metamorphosis. Often parasitic in adult phase. Attaches to fish and sucks essential nutrients.

Boneless Fish

Sharks: Usually marine, although some freshwater species are found. Skeletal structure made of cartilage, skin is covered with scales. Has a complex brain, spinal cord, sense organs including eyes, pressure sensitive organs which sense water movement, and electric current sensors. The digestive tract is complex. Sexes are separate, and fertilization is internal (that is, the eggs remain inside the female's body to receive the sperm). Most species carry the eggs internally until hatching.

Rays and skates: are generally flat in shape, feed on mussels and clams, and some have stings or the ability to shock prey.

Bony Fish

24000 freshwater and marine species with great diversity in size and shape. Most have bony skeletons, scales, fins along both the dorsal and ventral surfaces, and tails. Digestive systems are complex, with mouth, stomach, liver, kidney, ureters, and cloaca to digest nutrients and dispose of wastes. In general, fertilization is external, with the female laying hundreds to thousands of eggs over which the male squirts his sperm; the eggs are left by the parents to fend for themselves. Fish circulate their blood through an open system, from the two-chamber heart past the gills, through a primitive kidney for purification, and into the body. They have simple brains and nervous systems, with various sensory organs (eyes, surface sensitivity).

Amphibians

Frogs, toads, and salamanders. Most amphibians must reproduce by laying their eggs in water, so even if the adult forms are completely terrestrial, they are usually found near water. The embryos develop into a larval stage which resembles bony fish: two chambered heart, simple brain, gills for oxygen absorption, tail for movement. During metamorphosis, the tail is absorbed, arms and legs form, the heart coverts to three chambers, eyes and other sensory organs become adapted to terrestrial conditions.

Reptiles

Turtles, lizards, snakes, alligators. Reptiles are the first animals we consider (if you have been moving downward on this list) which are completely terrestrial and do not need to return to the water to reproduce. Their eggs have a leather shell which protects the embryo from water loss and predators. The adult forms have dry scaly skins which similarly control water loss. Reptiles have a three chambered heart which is more efficient than the amphibian heart; however, like all fish, amphibians, or protostomes, they are cold-blooded and cannot regulate their internal body temperature. They have complex nervous systems with brains and separate sensory organs like eyes and tongues which are sensitive to both tase and smell. Their complex digestive system helps them cope with their diet (they are primarily carnivorous, so they have to be able to break down complex proteins) and waste. Many are adapted to specific habitats (desert, near water, arboreal, high altitude).

Birds

All birds are adapted for flight, possessing wings (even if stubs used for swimming, as in penguins), feathers, and hollow bone structures which reduce weight. In order to sustain the high energy-expenditure levels required for flight, birds have four chambered hearts and very efficient lung systems. Their digestive system includes a crop in which food is temporarily stored before it is passed through the esophagus into the gizzard chamber of the stomach, where it is ground, and then subjected to digestive juices. Their brain is proportionally larger than that of reptiles, in order to support their reliance on vision for information, and on hearing — birds use song to identify themselves and mark territory. Many birds migrate, inhabiting one area during mating season and another during breeding. Birds are found in most terrestrial habitats.

Mammals

Mammals are divided into those that actually lay eggs (duck-billed platypus and spiny anteaters), thous that have pouches (marsupials), and true mammals, all of which have hair, mammary glands, and bear their young alive. All mammals have four-chambered hearts and solid bones, which allow them to grow larger and move faster than most other animal forms. They all have complex digestive systems and reproductive organs; in addition, many have complex behavioral patterns for everything from habitat choice to mating. Mammals have complex sensory organs for sight, hearing, touch, taste, smell, and balance, and proportionally larger brains. Many mammals are social, living in family groups, and their young have fewer instinctive reactions to their environment than most other organisms — which means that the young have to stay close the parents for long periods of training in survival and appropriate behavior.