SECTION II. 



THE CLASS OF ECHINODERMATA OR ECHINODERMS. 



SUB-SECTION I. 

 THE ECHINODERMATA CONSIDERED AS A CLASS. 



THE Echinodermata are radiate animals which have a 

 tough skin containing particles of carbonate of lime, or a 

 shell composed of calcareous pieces, which are movable, 

 or fixed together, and covered with tubercles or spines. 



The parts of their structure radiate from the mouth or 

 oral opening, and meet in the opposite pole or.ab-oral 

 region ; and in general there are, along certain of the 

 rays, regular rows of tubular suckers used in locomotion. 



The name Echinodermata comes from the Greek eckinos, 

 a hedgehog, and derma, skin, and is given to these ani- 

 mals because many of them are covered with spines, as 

 stated above, thus reminding us of the hedgehog (Fig. 

 135) of the fields. 



In the Radiates there is generally a reigning number 

 a number to which the parts conform. Tn the Echino- 

 derms this number is five ; that is, the parts are five, or 

 some multiple of five. 



The internal organs are quite well defined. In the 

 Holotlmrioids and Sea-urchins, the alimentary canal, form- 

 ing one or two turns, extends through the whole length 

 of the animal. In the Star-fishes the digestive cavity is 

 a sac with branching appendages, and with only one open- 

 ing. 



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