ECHINOPERMATA. 77 



Class III. Echinoidea. Sea-urchins. 



Body rounded or flattened. Twenty rows of calcareous dermal plates 

 make up a strong test, perforated by rows of tube-feet, and bearing 

 movable spines and pedicellarise. Tube-feet with suckers. Blood- 

 vascular system often well developed. The genital pores lie at the 

 vertex (periproct) of the shell on interradial genital plates one of which 

 is the madreporic plate; radial ocular plates alternate with the preceding. 

 The mouth of the gastrula persists as the anus. Development with a 

 Pluteus larva. Many occur fossil. * 



Order 1. Regularia. Spherical, with masticatory apparatus (Aristotle's 

 lantern) of five oral teeth, &c. Echinus melo, Toxopneustes, Stron- 

 gylocentrotus lividus, all with oral gills; Cidaris, without oral gills. 

 Calveria (Asthenosoma), a deep-sea form with flexible test. 



Order 2. Clypeastroidea, shield-urchins. Irregular, with central 

 mouth and masticatory apparatus ; around the aboral pole a five-leaved 

 ambulacral rosette ; anus excentric. Clypeaster ; Mellita with fenes- 

 trated shell. 



Order 3. Spatangoidea, heart-urchins. Irregular with excentric 

 mouth and anus; no masticatory apparatus. Spatangus purpureus 

 with four genital glands and openings. 



