STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 227 



of Grantia labyrinthica, and there is no possibility of doubt 

 as to the exact position of the pendent ovum. Fig. 36 repre- 

 sents an ovum of the same sponge which has presumably been 

 fertilised^ and has taken up the normal position for development, 

 just behind the layer of collared cells which lines one of the 

 flagellated chambers, as shown in figs. 5 and 13. 



For an account of the spermatozoa of the Heterocoelal must 

 refer the reader to PolejaeflP's work on the subject (34). 



Endothelial Cells. — Under this name I have previously de- 

 scribed (9) the more or less flattened cells which line, in a single 

 layer, the cavities in which the embryos develop. Fig 38 shows a 

 typical example of an embryo lying in a cavity lined by such 

 cells. Schulze (7) attributes to these cells a mesodermal 

 origin. For the present I adopt this view ; but I would like 

 to point out that they may possibly be ectodermal, for it 

 is easy to imagine that an ovum, after migrating through the 

 wall of the inhalant canal, in returning into the mesoderm 

 may push before it a portion of the ectodermal epithelium, from 

 which the embryo capsule might be derived. This, however, 

 is, in the present state of our knowledge, mere speculation. 



V. — The Classification of the Heteroccela Calcarea, 

 WITH Diagnoses of the Families and Genera. 



We are now in a position to apply the results of our anato- 

 mical investigation to the classification of the group. I have 

 found it necessary to forestall to a certain extent what I have 

 to say here about the classification, in order to satisfactorily 

 arrange my notes on the anatomy, but this was obviously 

 unavoidable. In the present section I propose not only to set 

 forth the classification of the group, and to give brief diagnoses 

 of the families and genera, but also to discuss, so far as 

 appears necessary, the questions of synonymy and nomencla- 

 ture. 



Order HETEROCCELA, Polejaeff (8). 



Diagnosis. — Calcareous sponges in which the collared cells 

 are confined to more or less well-defined flagellated chambers. 



