Phylum Brachiopoda (lampshells)
These animals superficially resemble clams but have upper and lower shells rather than left and right ones. All are marine. 230 species.
Eucoelomates: the Trochozoa
The following phyla frequently possess a juvenile stage called a trochophore larva. They all may be derived from an annelidlike ancestor, judging from their widespread metamerism (segmentation) and the frequent occurrence of paired nerve cords ventral to the gut.
Phylum Annelida
Annelids have a well-developed coelom, a soft body, and (as a rule) well-developed metamerism. Bristlelike structures (setae) used in crawling are often present. The group includes earthworms (class Oligochaeta), leeches (class Hirudinea), and the less familiar bristle worms (class Polychaeta). Some animals of uncertain position, here treated as classes of annelids, are sometimes made separate phyla: Echiura and Pogonophora (beardworms). Two minor groups within the polychaetes, Archiannelida and Myzostomida, are often considered independent classes. 8700 species.
Phylum Sipuncula (peanut worms)
These moderate-sized (about 3 cm/1.2 in) marine worms have a saclike body and a long proboscis. They may be simplified annelids. 250 species.
Phylum Mollusca
The second largest animal phylum, including snails and clams, mollusks typically have a hard shell and a soft body. Few traces of metamerism are observed, and the coelom is small; the main body cavity is part of the circulatory system. Some forms, such as octopuses and squid, attain considerable size. The seven mollusk classes are Aplacophora, Polyplacophora (chitons), Monoplacophora, Gastropoda (snails and slugs), Bivalvia (clams and allies), Cephalopoda (octopuses, squid, and allies), and Scaphopoda (tusk shells). 50,000 species.
Phylum Arthropoda
The body of an arthropod is covered with a hard, jointed skeleton. This is the largest animal phylum, partly because so many kinds of insects exist. Arthropods are abundant and successful in almost all habitats. The body remains segmented, and the coelom has been reduced. Three small, transitional groups are sometimes included in the Arthropoda and sometimes made independent phyla: Onychophora, Tardigrada, and Pentastomida. The classes of higher arthropods can be grouped into assemblages. Those with jaws (mandibulates) include the Crustacea (lobsters, crabs, and allies), the allied Myriapoda (centipedes and millipedes), and Insecta (insects). Arthropods with mouthparts called chelicerae include the Pycnogonida (sea spiders), Merostomata (horseshoe crabs), and Arachnida (spiders and their allies). 838,000 species.
Eucoelomates: the Deuterostomia
This large group includes all the remaining animals. The group’s name indicates that the mouth is produced secondarily, as a new formation, in embryological development.
Phylum Chaetognatha (arrowworms)
These animals are small, active, exclusively marine swimmers of uncertain relationship. 50 species.
Phylum Echinodermata
Echinoderms derive their name from their spiny skins. The group includes starfish and sea cucumbers. They are slow moving and rely mainly on small processes called tube feet for locomotion. All are marine. The five extant classes are as follows: Crinoidea (feather stars), Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins), and Holothuroidae (sea cucumbers). 6000 species.
Phylum Hemichordata (acorn worms)
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.
Animals in the Human Community
Ever since true humans appeared on earth, they have lived in close association with other animals. Throughout most of that time, humans existed as hunter-gatherers who depended on wild animals for food and clothing. They also found a source of aesthetic pleasure and spiritual strength in the animal life about them. Evidence of this exists in the remarkable paintings of Paleolithic people in the caves of southern France. The religious practices of Native North Americans involved the veneration of certain animals, such as the bear, wolf, and eagle, in an attempt to acquire the power, wisdom, courage, and speed that these animals supposedly possessed.
Human societies have continued to exhibit traces of these relationships. The aesthetic appeal of animals is expressed in the mosaics and frescoes of ancient Greece, Rome, and Egypt; in the paintings of the Renaissance masters; and in countless works of more recent artists. Mystical feelings toward animals are reflected in myths and folktales, moralistic fables involving animals, and anthropomorphic animal stories for children, as well as the adoption of animal names for cars, sports teams, and so on.
Despite highly developed technology and agriculture and the domestication of many species, modern humans still depend on wild stocks of a variety of other animals, such as fish and shellfish, for food. Unmanaged commercial exploitation, however, has greatly reduced this resource and has driven some species to the edge of extinction . In addition, wild animals provide recreation in the form of sport hunting and sportfishing, bird-watching, and visits to zoos and wildlife refuges.
As humans advanced from a hunting to an agricultural existence, relationships between humans and animals changed. Animals that preyed on livestock or that destroyed crops were reduced or exterminated, and the alteration of the landscape eliminated habitats for many other species. As humans became more concentrated in cities, however, animals such as rats multiplied and became important carriers of disease.
Humans domesticated some animals for food, clothing, power, and companionship. How this came about is a matter of speculation. Through protection and selective breeding, humans changed early domesticated forms into highly productive dairy and beef cattle, sheep, swine, and poultry. Also contributing to human welfare are the dogs, cats, white rats and mice, guinea pigs, and rhesus monkeys that medical research has used to gain an understanding of human physiology and to develop drugs and procedures to combat human illnesses.
As humans continue to spread across the earth, however, they encroach on or pollute the environments of many animals, reducing remaining habitats to smaller and smaller areas. Unless this trend is reversed, much of the world’s animal life faces extinction.