542 ECHINODERMATA ECHINOIDEA chap. 



is frequently left bare at low tide. It thus takes the place of 

 E. miliaris in the British fauna. An allied if not identical 

 species, S. purpuratus, is found in Puget Sound on the Pacific 

 coast. 



Other interesting genera of the Echininae are Echinometra, 

 Colobocentrotus, and Heterocentrotus. All possess large, thick 

 primaries, and all are elliptical in outline. In Echinometra the 

 primaries are pointed, and the long axis of the body makes an 

 oblique angle with the axis passing through mouth and madre- 

 porite. In Colobocentrotus and Heterocentrotus the axis passing 

 through mouth and madreporite is the short axis of the ellipse, 

 and the primary spines are very thick and triangular in section, 

 whilst the expanded ends of the secondaries form a closely set 

 armour between the bases of these. In Colobocentrotus the test 

 is markedly flattened on the under side, and this flattened area 

 is fringed with a circle of primaries ; but in Heterocentrotus there 

 are a few rows of primaries all over the test. These are tropical 

 genera and are found on the outer side of coral reefs, and they 

 require the cuirass of expanded secondaries to protect them 

 against the waves. 



Order II. Clypeastroidea {Cake-urchins). 



The " Cake-urchins " have only one representative in the 

 British area, and this is unsuitable for dissection on account 

 of its small size. We shall therefore select as type the " Sand- 

 dollar " Echinarachnius par ma (Pigs. 241, 242), which occurs 

 abundantly in shallow water on the east coast of North America. 

 As its popular name implies, this is an extremely flattened Sea- 

 urchin of nearly circular outline, so as to suggest a resemblance 

 to the silver dollar of North American currency. The peristome is 

 exceedingly small, and is placed in the centre of the lower surface 

 (Fig. 241), whilst the periproct is placed on one edge. The out- 

 line is not quite circular, for the periproct lies in a slight in- 

 dentation of the edge ; and this side is broader and of a lesser 

 degree of curvature than the opposite one, so that a secondary 

 bilateral symmetry is superimposed on the fundamental radial 

 symmetry common to all Echinoderms. A line drawn so as to 

 pass through the anus and the centre of the disc will divide the 

 animal into two similar halves ; the periproct of course lies in 



