100 



SPONGES 



In the above account of the canal system of the Heterocoela, a 

 Leucosolenia-like form, consisting of an Olynthus surrounded by numerous 

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

 ceeding is the more justified, since the 

 majority of Heterocoela, and especially 

 the genera Sycon and Leucandra, and 

 their allies resemble the Leucosoleni- 

 idae in just those characters of skeleton, 

 histology, and embryology in which the 

 latter differ from Clathrinidac. There 

 may be, however, amongst the Hetero- 

 coela forms which are to be referred 

 back to a Clathrinid ancestor which 

 has undergone modifications of the canal 

 system more or less parallel to those 

 which have been followed out above, 

 and though the Heterocoela have not 

 yet been studied from this point of 

 view it is highly probable that this is 

 the case. The genus Ascandra among 

 Clathrinidae, with its folded gastral 

 layer, represents a type of structure 

 which might easily serve as the starting- 

 point for the evolution of a hetero- 

 coelous canal system. The curious 

 genus Heteropegma of Polejaeff (1883), 

 for instance, which in its outer form 

 closely resembles a typical Clathrina, 

 composed of a network of tubes, seems 

 to be modified from a Clathrinid 

 ancestor. 



Skeleton. In the class Calcarea 



Vertical section' o^'the osculum of the skelet n is composed of spicules 

 Leucandra aspera, schematised; the thick of Carbonate of lime in the form of 



calcite. The skeletal elements are 

 typically quite separate one from 

 another, but if united into a con- 

 tinuous framework, as is known to 

 occur in at least one instance (Petro- 



stroma), the union is brought about by fusion taking place between 

 the spicules themselves, and not by means of spongin or any other 

 form of special cementing substance. No distinction can be drawn 

 in this group between megascleres (skeletal spicules) and micro- 

 scleres (flesh spicules). 



The calcareous spicules have a crystalline structure, and each 

 spicule, whatever its form, behaves optically as a single crystal 

 individual. Each spicule ray has an organic axial thread, and is 



oscular sphincter ; i 



of spicules ; ost, 



cavity ; tn.c, incurrent canal ; ex.c, ex 



current canal. Combined from several 



sections. 



