SPONGES 



(b) The instances of the chambers themselves being folded or 

 branched are numerous, and an extreme case is seen in the ear-like 

 form Euryplegma (Fig. 20, C, and Fig. 80). 



This condition is at first sight difficult to distinguish from the condi- 

 tion found in the Hyalonematidae, a family remarkable for the fact that 

 the chambers grouped round each excurrent canal are continuous with 

 one another at their apopyles, the gastral epithelium passing on without 

 interruption from chamber to chamber. In fact, each excurrent canal 

 in Hyalonema might be thought to be a single, branched chamber, were it 

 not for the important difference that the subgastral layer and the gastral 

 membrane extend, as has been said, into it. This feature at once dis- 

 tinguishes the excurrent sinuses from branched chambers, since no 



d.nt. 



"diet 



sgtr 



app 



G.C 



FIG. 



Section of the wall of Euryplegma auriculare, F.E.S. All spicules are omitted except the 

 dictyonalia. x 25. (After F. E. Schulze.) diet, the dictyonal framework formed by union of 

 the principalia one to another. . 



such extension of the inner layers of the body wall into the lumen 

 of the chambers ever occurs. The condition found in the Hyalo- 

 nematidae would appear therefore to represent a fusion of chambers 

 primitively distinct, or more probably still a condition where the multi- 

 plication of chambers by fission has stopped short of completion. 



The uniform and simple structure of the body wall in Hexactinellid 

 sponges makes it easy in these forms to determine in any specimen the 

 relations of the gastral cavity, since the anatomy of the young forms (Fig. 

 76) shows clearly that the subgastral membrane, through which the water 

 passes after issuing from the apopyles and traversing the subgastral frame- 

 work, is its boundary. Hence any space which is limited by, or borders 

 upon, the subgastral membrane, must be morphologically the gastral 

 cavity. We have already described the series of form modifications 

 whereby the gastral cavity may become greatly widened, and finally, in 



