These simple, wormlike marine animals have a few characteristics linking them to chordates. Among these are traces of what may be a notochord (see discussion under Phylum Chordata) and a dorsal nerve cord, as well as a system of gill slits. The larval body of the worms, however, indicates a link to the echinoderms. 230 species.

Phylum Chordata

Chordates include vertebrates (animals with backbones) and some related invertebrates. At some time in their lives, all possess a stiff rod, called a notochord, that lies above the gut. In vertebrates, a series of bones (vertebrae) replaces the notochord.

Two subphyla of invertebrate chordates exist. The Tunicata (tunicates, or sea squirts) become highly modified as adults and attach to a substrate, feeding with gill slits. Only the free-swimming larvae retain a notochord. All are marine, and about 1300 species exist. The Cephalochordata (lancelets) resemble very simple fish. They live in the sea and extract food from the water with their gill slits. About 25 species exist.

The remaining Chordata belong to the subclass Vertebrata and have a backbone. Half of the 42,000 species are fish. Thus, although the group is very successful, it is outnumbered by arthropods and mollusks. Seven classes are generally recognized. The old class Pisces (fish) has long since been broken down into three classes: Agnatha (jawless fish such as lampreys), Chondrichthyes (cartilaginous fish such as sharks and rays), and Osteichthyes (bony fish). The class Amphibia contains such semiaquatic forms as salamanders, toads, and frogs. The class Reptilia is better adapted to life on land; it includes turtles, lizards, and crocodilians. The class Aves (birds) is noteworthy not only for flight but also for warm blood and an insulating layer of feathers. The class Mammalia is characterized by the possession of hair and of mammary glands that secrete milk. This group also is warm-blooded.

Animals in the Ecosystem

Animals have diversified so as to occupy many ecological niches. Herbivores form a basic link in the food chain, or food web. They are eaten by carnivores, which in turn are eaten by other carnivores. Many animals are parasites or scavengers or feed on sediments. Every animal has adaptations, such as sense organs and teeth, that enable it to find and utilize food. Some use a wide variety of foods; others specialize. Features such as camouflage and protective shells enable them to cope with predators.

Animals compete with one another for scarce resources, including food and a place to live. The interactions between predator and prey have involved a coevolution of these different parts of the ecosystem—that is, the evolution of prey is affected by the evolution of predators. Parasites, which are harmful but usually do not kill their hosts, must be able to overcome defensive adaptations. On the other hand, mutualists have cooperative arrangements with other organisms. Plants and animals can also cooperate, in the sense that food is provided by plants, and various animals effect pollination or seed distribution.

A great variety of strategies enable animals to survive and reproduce. When food in an ecosystem is abundant, the animals involved generally tend to reproduce in quantity. When intense competition for food is involved, however, the animals have to develop a more effective use of resources and tend to care for the young for a prolonged period. Where the ecosystem is stable, as in the Tropics, competition leads to complex life cycles and intricate adaptations, both structural and behavioral.

Animals often move from place to place. This enables them to locate food, avoid predators, and reproduce effectively. Through migration, birds and other animals can feed and reproduce where food is seasonally abundant. The movement of animals is limited only by the food supply, the terrain, and their individual capabilities for locomotion, and each local area and habitat has tended to evolve its own distinctive fauna.