CHAPTER XVII 



THE ANATOMY AND PHYSIOLOGY OF ECHINO- 



DERMS WITH PARTICULAR REFERENCE TO THE 



SEA-URCHINS 



General Form of the Body. — In the echiiioderms the body 

 has a radial arrangement of parts ; i.e. there are more than 

 two similar parts arranged around the chief axis of the body, 

 whereas in bilaterallj' sjmimetrical animals, like ourselves, 

 there arc only two such parts. In echinoderms the numljer of 

 such parts is nearljr constant; namely, five (Fig. 229). Now 

 a radial arrangement of parts is a characteristic of an attached 

 organisnr — a hj^droid sea-anemone, sunflower, or tree — as 

 a bilateral arrangement is of a locomotor organism. Conse- 

 quently it is tolerably certain that the ancestors of echinoderms 

 were attached animals. Among modern echinoderms such 

 attached animals are, indeed, found in the groups of Crinoids 

 or Sea-lilies, a group that was formerly more abundant than 

 it is now (Fig. 2-52). Also in earlier geologic times echino- 

 derms were represented Y)y two or three other classes now 

 totally extinct, of which some were stalked. It is possible 

 that from such sessile organisms the free-li\dng echinoderms 

 of to-day sprang. 



In the bod}' of any echinoderm we recognize a main axis 

 with two rjuite different poles, i.e. at the two entls of the axis 

 the body is riuite different. That pole where the mouth 

 usually lies is called oral : the opposite pole ahoral. Typically 

 there are five ecjual parts arranged about this main axis. In 



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