SPONGES 



IJ 3 



cither (a) to a folding of the chamber layer as a whole, or (1>) to 

 the folding and branching of the individual chambers. 



(a) The simplest cases of the folding of the chamber layer result 

 in a type of canal system which reminds us of what has been de- 

 scribed above in the calcareous sponge, genus Leucilla (cf. Figs. 70 

 and 78). Short excurrent bays are formed into which the chambers 

 open, the latter being disposed into radiating groups round each 

 bay. Further development of this process of folding leads to the 

 formation of long branched excurrent canals, and the whole canal 

 system approaches very nearly to the type seen in Leucons. The 

 extent to which the folding of the chamber layer affects the other 



..fc. 



Sdtr 



Fir:. 77. 



'Section of the body wall of Eiiplr.ctella aspergillum, Owen. xllO. (After F. E. Schulze.) 

 f.c, floricomes (i.e. a form of hexaster) ; prc, principalia ; ast, parenchymal hexasters ; prp, 

 prosopyles ; app, apopyles. Other letters as in Fig. 70. 



layers of the sponge varies considerably. In the simplest cases the 

 subdermal trabecular layer alone is affected (Fig. 78), and extends 

 down into the interspaces between the folds of the chamber layer. 

 In most cases, however, the subgastral trabecular layer is folded 

 with the chamber layer, so that it extends into the excurrent 

 canals, while the subgastral membrane remains unaffected, and 

 either stretches across the openings of the excurrent canals (Fig. 79), 

 or is interrupted at these spots. But in extreme cases, as seen in 

 the family Hyalonematidae, the subgastral membrane shares in the 

 folding of the chamber layer and forms a lining to all the excurrent 

 canals. In no case does the subdermal membrane take any share 

 in process of folding. 



