ECHINOIDA (SEA-URCHINS). 



In the sea-urchins the body is spherical, heart-shaped, or 

 disc-like, and the ambulacral areas extend, like meridians, 

 from oral to anal regions. In short, sea-urchins are easiest 

 compared with starfishes, if we imagine the arms of the 

 latter bent backwards until they meet above. In this way 

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

 area, while the reproductive openings, by the union of the 

 arms, would be forced into a position between the oculars, 

 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 by the union of those of the same interradius. 



The Echinoida 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. 



