ECHIXODKK.MATA 



97 



distance from the mouth, which is surrounded by a rather firm 

 membrane. From the mouth itself project the tips of five 

 teeth, radially arranged. The anus is at the opposite pole of 

 the animal, and is provided with skeletal plates more or less 

 regularly arranged, around which are two concentric series of 



FlG. 87. Strongylocentrotus, aboral aspect, showing spines and tube feet. (From Parker 



and Haswell's Manual.) 



regular plates, five in each series. The inner five are called the 

 genital plates, for each is pierced by an opening which marks 

 the orifice of the duct from one of the five germ-glands. One 

 of these plates is pierced by a large number of pores, and this 

 is the madreporic plate, corresponding to the same structure in 

 the starfishes, and communicating with the water vascular sys- 

 tem, the structure of which is much the same as the type 

 already described. The outer five plates, which alternate in 

 position with the genital plates, are called ocular plates, for a 



H 



