SPONOES 





^>0 



In the above account of the cAual system of the Hftfrocoela, a 

 IruotWrnui-like form, consisting of an ()/ynMi« surrounded by numerous 

 radial diverticula, has been taken as the starting-point, and this pro- 

 ceeding is the more justified, since the 

 majority of Hftavco^la, and especially 

 the genera Si/com and Lfttcuiidrn^ and 

 their allies resemble the LrucosoUni- 

 tdae in just those chan^cters of skeleton, 

 histology, and embryology in which the 

 latter ditfer fn>m Clathrinidof. There 

 may W, however, amongst the Hftrro- 

 rofla forms which are to l^e referred 

 back to a Clathrinid ancestor which 

 has undergone moditications of the canal 

 svstem more or less jxarallel to those 

 which have l»een followed out alK>ve, 

 and though the Hfteronxht have not 

 yet Weji studied from this point of 

 view it is highly probjible that this is 

 the case. The genus .4.<r<iMi/ni among 

 Claihrinuiar, with its folded gastnU 

 layer, represents: a tyjv of structure 

 which might easily serve as the starting- 

 point for the evolution of a hetero- 

 coelous canal system. The curious 

 genus H(tfr<^>fxijma of rolojaetf (1S83), 

 for instance, which in its outer form 

 closely resembles a typical Clathriitu, 

 comjKJseil of a network of tubes, seems 

 to be modified from a Clathrinid 

 ancestor. 





' "6: 







+~*^-— iMf. 



:\ 



' Jv^v 



Vert 

 Lncam. 



ruriviit ciM»l. 

 s<x-tions. 



Fic. n. 



CV)mhin*»l from several 



Shit ton. — In the class C;iloai-oa 

 the skeleton is comjx^setl of spicules 

 of carlx»nate of lime in the form of 

 oaloite. The skeletal elements are 

 typically quite se}Vir^\te one from 

 another, but it uniteti into a con- 

 tinuous framework. a5 is known to 

 occur in at least one instance {Pdit^- 

 ftn>i)ui\ the union is brought about by fusion takini: place between 

 the spicules themselves, and not by means of sjx»ngin or any other 

 form of sjH^cial cementing sulvstance. No distinction can be drawn 

 in this group Wtween megasoleres (skeletal spicules) and micro- 

 sderes (tlesh spicules). 



The calcareous spicules have a crystalline structure, and each 

 spicule, whatever its form. lM?haves optically as a single crystal 

 ii.ilividnil. E.*ich spicule my has an organic axial thread, and is 



II 



