534 



ECHINODERMATA ECHINOIDEA 



The auriculae are incomplete and consist only of pillars 

 arising from the interambulacral plates. The ambulacral pore- 

 plates remain disunited, and the pores are arranged in a single 

 vertical series ; hence the ambulacra are very narrow. The inter- 

 ambulacral plates each bear one large primary spine surrounded 

 by several circles of secondaries. No ophicephalous or trifoliate 

 pedicellariae are to be found, and the gland of the gemmiform 

 pedicellaria is placed inside the concavity of the blade. 



The Cidaridae are in many respects the most primitive of the 

 six families living. They are distributed all over the world, and 

 chiefly inhabit deep water. No two naturalists agree as to how 

 they are to be divided into genera. Mortensen, 1 who takes the 



anus 



penproct 



anus 



Fig. 236. Figure showing periprocts of A, Cidaris ; B, Echinus, x 1. avib, Ambulacral 

 plate ; g.o, genital opening ; g.p, genital plate ; inter, interambulacral plate ; tn.p, 

 madreporite ; oc, ocular pore. 



structure of the pedicellariae as his principal guide, recognises 

 fourteen genera. Others (as for instance Bell) have been 

 inclined to attribute nearly all the living species to one 

 polymorphic genus, Cidaris, finding all attempts to divide 

 the genera from one another frustrated by the discovery 

 of transitional forms. Goniocidaris (Fig. 237), however, dis- 

 tinguished by its comparatively broad poriferous zones, by 

 bare places in the middle line of both radii and interradii, and 

 by deep pits on the lines of suture of the plates, is by general 

 consent distinct. This genus is confined to the Eastern Pacific, 

 but from British waters three species of Cidaris have been 

 recorded, only one of which, C. (Dorocidaris) papillata, is at all 

 common. It is found in water from 100 to 500 fathoms in depth 



1 Loc, cit. 



