ANIMALS (Part 4 of 4)

The Largest Group of Animals

The phylum Arthropoda ("jointed foot") has the largest number of species. In fact, about 90 percent of the million or more species living on the Earth today are arthropods. The insects total more than 800,000 species. Other arthropods include the centipedes and millipedes; the arachnids (spiders, scorpions, ticks, mites); and the crustaceans (barnacles, crabs, crayfish, lobsters, shrimp, water fleas). Obviously, the arthropod body plan has been highly successful. The members of this great phylum live on land, in fresh water, and in salt water. They can walk, fly, burrow, and swim. This is the only invertebrate group with jointed appendages (legs, feet, and antennae).

Arthropod, animal of the phylum Arthropoda comprising invertebrates with external skeleton, segmented body, and jointed appendages.

Arthropods, like molluscs, wear a supporting framework, or exoskeleton, on the outside of the body. Much more highly developed than the heavy, clumsy shell of the clams and snails, it is made of a substance called chitin. Rigid, waterproof plates of chitin are joined by thin, flexible membranes of chitin so that the animal can move freely and quickly. The muscles are attached to the inner surface of the armour Many important structures are connected to the outer surface. For example, the wings, legs, jaws, and antennae of the insects are all made of chitin and are attached to the outer skeleton. The body is divided into sections, or segments.

Spiny-Skinned Animals

One phylum with the characteristics of several others is the Echinodermata ("spiny-skinned"). All members of the group, which includes starfishes, sea urchins, holothurians (sea cucumbers), and crinoids (sea lilies), live in salt water some in the shallow shoreline waters, others in the ocean depths. The young, called larvae, have bilateral symmetry, but the adults have radial symmetry, like the coelenterates. These are the most primitive creatures having an endoskeleton, or skeleton that is embedded in the flesh. It consists of a mesh work of plates that are made of calcium. The plates are joined by connective tissue and muscles. Spines project from these plates.

Animals with Backbones

At the top of the animal kingdom is the phylum Chordata. This phylum consists of two groups of primitive chordates the tunicates and the cephalochordates and the main group of the vertebrates.

The major subdivisions, or classes, of the vertebrates are the fish, amphibians, reptiles, birds, and mammals. Members of this phylum possess the following structures at some period of their life, either as embryos or as adults:

Notochord. This is an internal supporting rod extending the length of the body. It is found in the embryos of all chordates, including human beings. Only the most primitive forms, such as the amphioxus, or lancelet, the lamprey, and the hag fish, retain it as adults.

Remnants of the notochord are also present in sharks. In the higher chordates, such as amphibians, reptiles, birds, and mammals, the notochord is replaced during development of the embryo by a bony column of vertebrae, which gives the column and the animal flexibility.

Nerve tube. This lies in the mid line of the body on the top (dorsal side) of the notochord. In the annelid worms and the arthropods the main nerve is solid and lies on the underside (ventral side). In most chordates the forward end of the nerve tube forms a brain; the remainder is the spinal cord.

Pharyngeal gill slits or pouches. The lower chordates, such as the fish, breathe through openings in the side of the neck in the region of the pharynx. The embryos of the higher chordates have these slits, but they disappear in the adult.

Primitive Chordates

Amphioxus (or lancelet), a fish-shaped sea animal; about 2 in. (5 cm) long; pinkish white; several species known; classed in the phylum Chordata, subphylum Leptocardia.

The amphioxus is characteristic of the most primitive chordates. This animal is a laterally compressed, semitransparent sea dweller about 4 inches (10 centimetres) long. Scientists believe that it may be one of the ancestors of the vertebrates. It has a notochord and a tubular nerve cord along the back. It has no well-developed brain, however, and only traces of eyes and ears. Pigment spots along the body are sensitive to light. The pharyngeal gill slits strain food from the water. The tunicates, or sea squirts, and acorn worms are other primitive chordates.

Lampreys and hag fishes are the most primitive of the true vertebrates. They have a notochord. The skeleton is composed of cartilage. They lack jaws and paired limbs.

Mammals

Mammals differ from other vertebrates in that they have bodies that are covered with hair at some period of their lives. They are warm-blooded, meaning that their body temperature is largely unaffected by the temperature of the air or water in which they live.

The females have milk glands to feed their young. Whales, as noted earlier in this article, dolphins, and porpoises are the most unusual-looking mammals because they resemble fishes. The most primitive mammals are the egg-laying platypus and the echidna, or spiny anteater. The marsupials, which incubate their unborn offspring in a pouch for a time, are also considered to be somewhat primitive. The remaining mammals transmit nourishment to their unborn young through a placenta and give birth to fully developed offspring.

Assisted by J. Whitfield Gibbons, Senior Research Ecologist and Professor of Zoology, Savannah River Ecology Laboratory, University of Georgia.

BIBLIOGRAPHY FOR ANIMAL

Books for Children

Alday, Gretchen. Devoted Friends: Amazing True Stories About Animals Who Cared (Betterway, 1990).

Aylesworth, T.G. Animal Superstitions (McGraw, 1981).

Gabb, Michael. Creatures Great and Small (Lerner, 1980).

Hirschi, Ron. Who Lives In the Forest? (Dodd, 1987).

Hutchins, R.E. Nature Invented It First (Dodd, 1980).

Lauber, Patricia. What's Hatching Out of That Egg? (Crown, 1979).

Lopshire, Robert. The Biggest, Smallest, Fastest, Tallest Things You've Ever Heard Of (Houghton, 1991).

Lurie, Alison. Fabulous Beasts (Farrar, 1981).

McCauley, J.R. Animals and Their Hiding Places (National Geographic, 1986).

McGrath, Susan. Saving Our Animal Friends (National Geographic, 1986).

Patent, D.H. Sizes and Shapes in Nature What They Mean (Holiday House, 1979).

Pope, Joyce. Do Animals Dream? (Viking, 1986).

Prince, J.H. How Animals Move (Elsevier/Nelson, 1981).

Pringle, L.P. Feral: Tame Animals Gone Wild (Macmillan, 1983).

Schulz, C.M. Charlie Brown's Super Book of Questions and Answers About All Kinds of Animals (Random, 1976).

Sunden, Ulla, ed. Remarkable Animals (Guinness Books, 1987).

Sussman, Susan and James, Robert. Lies (People Believe) About Animals (Whitman, 1987).

Windsor, Merrill. Baby Farm Animals (National Geographic, 1984).

Books for Young Adults

Adamson, Joy. Born Free (Pantheon, 1987).

Adamson, Joy. Living Free (Harcourt, 1961).

Argent, Kerry. Animal Capers (Doubleday, 1990).

Baker, M.L. Whales, Dolphins, and Porpoises of the World (Doubleday, 1987).

Burton, Maurice. Cold-Blooded Animals (Facts on File, 1986).

Burton, Maurice. Warm-Blooded Animals (Facts on File, 1987).

Burton, Maurice and Burton, Jane. The Colorful World of Animals (Longmeadow, 1975).

Gibbons, Whit. Their Blood Runs Cold: Adventures with Reptiles and Amphibians (Univ. of Ala. Press, 1983).

Herriot, James. All Creatures Great and Small (St. Martin's, 1972).

Herriot, James. All Things Bright and Beautiful (St. Martin's, 1974).

Herriot, James and others. Animal Stories, Tame and Wild (Sterling, 1985).

Kohl, Judith and Kohl, Herbert. Pack, Band, and Colony: The World of Social Animals (Farrar, 1983).

Milne, Lorus and Milne, Margery. A Time to be Born (Sierra Club, 1982).

National Geographic Book Service. Wild Animals of North America, rev. ed. (National Geographic, 1987).

Nowak, R.M. and Paradiso, J.L. Walker's Mammals of the World (Johns Hopkins Univ. Press, 1983).