ECHINODERMS. 281 



latter bent backwards until they meet above. In this way 

 the terminal eye-spots * would be brought next to the anal 

 area, while by the union of the arms the reproductive open- 

 ings would be forced into a position between the ocular 

 plates, and the madreporite would become pressed against 

 one of the reproductive (genital) plates. 



All of the plates are firmly united to one another, while 

 the spines are freely movable, and share, with the ambu- 

 lacra, locomotor functions. The mouth is armed with five 

 teeth, and to aid in the movement of these a calcareous 

 framework is found just inside the mouth, known from its 

 first describer as Aristotle's lantern. In some, as in our 

 common urchins, this framework and its muscles are com- 

 plicated. From the mouth the tubular alimentary canal 

 pursues a winding course (usually folding on itself) to the 

 vent. Hepatic caeca, gastric pouches, and branchial trees 

 are lacking. The reproductive organs become fused into 

 five lobes bv the union of those of the same interradius. 



*/ 



The Echinoidea are divided into three orders: 



ORDER I. REGULARIA. 



In these, which embrace the more common urchins, the 

 mouth is at one pole, the vent at the other ; and the body 

 is approximately spherical. 



ORDER II. CLYPEASTROIDEA (Sand-cakes). 



In the 'sand-cakes' and ' sand-dollars' we have urchins 

 in which the test is disc-shaped and the ambulacra are con- 

 fined to the upper surface. The mouth is in the centre of 



* In only a few sea-urchins are the 'eye-spots' known to be 

 visual organs; the opening in the ocular plate is for the passage 

 of the terminal tentacle of the ambulacra! system. 



