PHYLUM ECHINODERMATA 



207 



Per-iproci- 

 Madrepor/fe 



Ocular- plof-e 



W' 



Jf7-f-efambu/acrc,/ . Ambu/ccrc^/ 



p/afes ^ p/afes 



io/ 



rainate in ocular and genital plates, respectively, leaving only the periproct 

 as aboral surface. The mouth of the starfish is simply an opening in the 

 center of a soft perioral membrane; in the sea urchins, however, it is 

 provided with five converging teeth. These are set in a complicated 

 skeletal box pentagonal in shape 

 and known as an Aristotle's lantern 

 (Fig. 120). This is made up of nu- 

 merous ossicles, lies \vithin the body, 

 and contains muscles which move the 

 teeth. The food of sea urchins con- 

 sists of algae, which they remove from 

 the surfaces of rocks with their teeth. 



Respiration in sea urchins usually 

 takes place by ten branched pouches 

 arranged in a circle around the mouth. 

 The tube feet are also said to be 

 respiratory. The latter may be 

 exceedingly long if the spines on the 

 surface of the animal are long, since 

 they reach beyond the spines. The 

 tube feet are used both in loco- 

 motion and in holding to surrounding 

 objects. In locomotion the spines are 

 used to prevent the pull of the tube 

 feet from rolling the animal over and 

 also as levers to help pry the animal 

 onward. The pedicellariae of sea 

 urchins are on a stalk and usually 

 have three jaws. 



Sea urchins differ in the length 

 and number of the spines. The cake 

 urchins and sand dollars are exceed- 

 ingly flat forms ^^^.th numerous very 



small and short spines ; they are found B, the oral surface, with the perioral 

 ^■^ ««„J u u u • iU .„ membrane torn loose for about two-thirds 



on sandy beaches, burying them- ^^ ^,3 attachment. Dried pedicellariae 



selves just under the surface of the are still attached to this membrane. 



sand as the tide goes out but mov- 

 ing about on the sand after the tide has returned. 



239. Holothurioidea. — The sea cucumbers, which make up this class, 

 differ from other echinoderms in the fact that they are greatly elongated 

 along the oral-aboral axis, the mouth being at one end and surrounded 

 by branched tentacles, while the anal opening is at the opposite end of 

 the body (Fig. 121). The body wall is muscular and possesses few and 



B 



Fig. 119.— Dried 

 of the genus Arbacia. 

 ment of plates. A, 



Afou/h 



shell of a sea urchin 



Shows the arrange- 



the aboral surface. 



