86 ZOOLOGY. 



ambulacral plates. In the other five zones which 

 alternate with the ambulacral rows, the plates are larger, 

 and are not perforated. The shell is increased in size by 

 additions made to the edges of each individual plate. 

 This is effected by a delicate membrane which covers the 

 test, and dips in between the edges of the various plates, 

 adding calcareous matter to each so evenly as to preserve 

 the shape of the whole. 



Besides the plates which occur in zones, or lunes, there 

 are others at the base and apex of the test. The mouth 

 is surrounded by a leathery membrane which contains a 

 number of small calcareous pieces called the oral plates ; 

 at the other extremity are similar anal plates. At the 

 apex of the test, and surrounding the anal opening, there 

 is a disc composed of a series of plates, termed the genital 

 and ocular plates. There are five genital plates, which 

 are larger in size than the ordinary plates, and pentagonal 

 in shape. Each of tliem is perforated by a duct from the 

 genital organs. One of these genital plates, larger than 

 the others, is furnished with a protuberance which con- 

 tains a large number of very minute openings. Through 



FIG. 36. TEST AND JAWS OF SEA-URCHIN, WITH THE SPINES REMOVED. 



these the water-vascular system communicates with the 

 exterior. The apertures are so small, that the water in 

 passing through them is freed from any particles of sand, 

 &c., with which it may be mixed. This protuberance is 



