ECHINOIDEA. 



37 



of the ambulacral areas, are five smaller, heart-shaped or pentagonal 

 plates, which are known as the " ocular plates," each of which is 

 perforated by a minute pore lodging the rudimentary sense-organ 

 (" ocellus ") in which the radial ambulacral nerve terminates. 



The anus in the Echinoids is situated in a membrane ("peri- 

 proct "), which is more or less largely hardened by the development 

 in it of small calcareous plates, which may be wholly irregular, or 

 may form one or more regular cycles (fig. 243, an). As previously 

 seen, the periproct in the " Regular " Sea-urchins is placed in the 

 centre of the apical disc (fig. 243, a), whereas in the "Irregular" 

 Urchins it has no connection with this structure, and is wholly 



Fig. 243. — a, Apex of the shell of Cidaris imperialis (Recent), enlarged, after Van der Hceven ; 

 B, Apex of the shell of Micraster (Cretaceous), showing the condition of the apical disc in the 

 " Irregular" Echinoids, after Zittel. an, Anus, surrounded by the plated "periproct " ; g, One 

 of the genital plates ; o, One of the ocular plates ; 7iia, Madreporite ; bi, the two posterior am- 

 bulacral areas, constituting the "bivium." 



excentric. The mouth in the Echinoids, like the anus, is situated 

 in a coriaceous membrane ("peristomial " membrane) hardened by 

 plates and granules of lime. Hence in macerated or fossil speci- 

 mens the under surface shows, in place of the true mouth-opening, 

 a wide vacant space ("peristome") originally occupied by the per- 

 istomial integument. The peristome may be rounded or oval, or 

 bilabiate, or, very commonly, pentagonal, often with incisions at the 

 angles for the accommodation of the so-called " oral gills." 



The internal skeleton of the Echinoids is represented by the so- 

 called "auriculas" of certain types. These are calcareous arches 

 which are ambulacral or " radial " in position, and spring from the 

 inner surface of the lower edge of the test, just where the imper- 

 fectly calcified peristomial membrane begins. Each forms an arch 

 over one of the radiating ambulacral vessels ; and the auricles cor- 

 respond, therefore, with the so-called "ambulacral ossicles" of the 

 Star-fishes. 



