. Principles of modern biology. Biology. The Animal Kingdom - 661 Fig. 32-33. Echinoderms, rep- resenting the five classes. A, a sea urchin, Class Echinoidea; B, a sea lilly, Class Crinoidea; C, a sea cucumber, Class Holo- thuroidea; D, a brittle star, Class Ophiuroidea; and E, a common starfish, Class Asteroi- dea. (From General Zoology, by Miller and Haub. Holt, Rinehart and Winston, Inc.). Embedded in the body wall there is an endo- skeleton, consisting of a number of hard calcareous plates, the ossicles; and projecting outward from the body there are many cal- careous spines. Collectively
. Principles of modern biology. Biology. The Animal Kingdom - 661 Fig. 32-33. Echinoderms, rep- resenting the five classes. A, a sea urchin, Class Echinoidea; B, a sea lilly, Class Crinoidea; C, a sea cucumber, Class Holo- thuroidea; D, a brittle star, Class Ophiuroidea; and E, a common starfish, Class Asteroi- dea. (From General Zoology, by Miller and Haub. Holt, Rinehart and Winston, Inc.). Embedded in the body wall there is an endo- skeleton, consisting of a number of hard calcareous plates, the ossicles; and projecting outward from the body there are many cal- careous spines. Collectively the ossicles and spines, plus a system of interconnecting mus- cles and strands of connective tissue, give great strength to the body wall. Usually the ossicles are about the size of very small pebbles, as in the common starfish, or they may be of microscopic size, as in the sea cucumbers. The spines also display con- siderable variation. Common sea urchins (Fig. 32-33) have strong pronglike spines, usually somewhat less than an inch long; but one semitropical genus (Diadema) has sharply pointed poison spines that may extend more than nine inches out from the surface of the body. On the other hand, the spines of many echinoderms are slender and delicate, as in the sea cucumber (Fig. 32-33). The symmetry of the echinoderms is another unique identifying feature. Super- ficially the adults display radial symmetry, a development that probably is related to the sedentary history of the group. The body always possesses a central part, the central disc, where the digestive tract is found. And surrounding the central disc there are five radially arranged body sections from which, in many species, more or less distinct arms (rays) project (Fig. 32-34).. 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 Marsland, Dougla
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