102 



THE SKELETON OF ECHIXODEEMS. 



mouth occupies the base of the test ; the opening is of a cir- 

 cular or decagonal form, in which a complicated mechanism of 

 five jaws and five teeth, with their muscles, are lodged (figs. 

 190 and 191). The anus in this group opens at the vertex of 

 the test ; the opening is surrounded by a circle of ten plates, 

 five of which are perforated to give passage to ducts from the 

 genital organs, and called ovarial plates, and five are per- 

 forated for lodging the eyes, and called ocular plates. The sur- 

 face of the ambulacral and interambulacral plates is covered 

 with tubercles of various sizes, in general raised upon prominent 

 eminences, the tubercles having a round smooth head, to which 

 a spine with a concave base is fitted and moved by muscles ; 

 the entire surface of the test and spines is covered by an or- 

 ganised skin ; the skeleton therefore is enclosed in mem- 

 branes, participating in the life and growth of the animal, 

 and forming an integral part of the urchin. 



In the ASTEEIADJE, or sea stars (figs. 36 and 373), a similar 

 complicated skeleton exists, with this difference, that the ambu- 

 lacral and interambulacral arese, instead of being united to 

 form a hollow case, are stretched out into rays, at the ex- 



Fig. 72. Apiocrinus rotunda. Fig. 73. Encrinus moniliformi 



