. Animal biology. Zoology; Biology. 204 METAZOAN PHYLA 234. Specializations.—Specialization or adaptation is the develop- ment of structures which fit an animal to perform certain particular functions or to meet certain peculiar conditions in the environment. The echinoderms show some of the most marked examples of specializa- tion to be found anywhere in the animal kingdom. Among these are (1) the entire water-vascular system, (2) the spines and plates which form the exoskeleton, (3) the pedicellariae, (4) the dermobranchiae, and (5) the Fig. 116.—The 20-rayed sunflower star, Pyc
. Animal biology. Zoology; Biology. 204 METAZOAN PHYLA 234. Specializations.—Specialization or adaptation is the develop- ment of structures which fit an animal to perform certain particular functions or to meet certain peculiar conditions in the environment. The echinoderms show some of the most marked examples of specializa- tion to be found anywhere in the animal kingdom. Among these are (1) the entire water-vascular system, (2) the spines and plates which form the exoskeleton, (3) the pedicellariae, (4) the dermobranchiae, and (5) the Fig. 116.—The 20-rayed sunflower star, Pycnopodia helianthoides (Brandt). Twenty to twenty-four rays are produced, usually in even numbers, although an occasional specimen is found with an odd number. Very young individuals may have as few as six rays. This species is one of the largest of starfishes, often attaining a diameter of 2 or more feet, and is common from California to Alaska. {Photograph by 0. Wade.) 235. Classification.—The phylum Echinodermata (e kl no der' ma ta; G., echinos, hedgehog, and dermatos, pertaining to skin) is divided into five classes: 1. Asteroidea (as ter oi'de a; G., aster, star, and eidos, form).—The starfishes. 2. Ophiuroidea (6 fi u roi'de a; G., ophis, serpent, ou7-a, tail, and eidos, form).—The brittle stars and serpent stars. 3. Echinoidea (ek i noi 'de a; G., echinos, hedgehog, and eidos, form).— The sea urchins and sand dollars. 4. Holothurioidea (hoi o thu ri oi' de a; G., holothourion, water polyp, and eidos, form).—The sea Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Wolcott, Robert Henry, 1868-1934; University of Nebraska (Lincoln campus). Dept. of Zoology. New York ; London : McGraw-Hill Book Company, Inc.
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