STUDIES ON THE COMPAEATIVE ANATOMY OP SPONGES. 47 



appearance in the membrane, which h, after all, only an 

 extension of the delicate protoplasmic collars of the collared 

 cells, and it is well known how plastic and changeable such 

 structures are. 



The main point, however, which I wish to establish concern- 

 ing Sollas's membrane in the present article is its coexistence 

 with the flagella of the collared cells, which has not hitherto 

 been proved. I wish also to support Bidder's view that its 

 function is the retention of food particles, which function 

 would be most effectually fulfilled by its stretching completely 

 across the prosopyles, and less so in cases where the prosopyles 

 are too wide to admit of this. 



I have now to describe the structure of the ova, which are 

 remarkable for their great complexity. In my third study on 

 the comparative anatomy of sponges I have described how the 

 mature ova of Grantia labyrinthica migrate through the 

 walls of the inhalant lacunae and remain suspended therefrom, 

 each by a distinct peduncle, awaiting fertilisation by the sper- 

 matozoa which enter the sponge with the incoming stream of 

 water. This is probably the mode in which fertilisation is 

 effected in most if not in all sponges. Fig. 2 represents a 

 mature ovum of Halichondria panicea suspended by a 

 peduncle from the wall of a lacuna. I have been unable to 

 prove that the lacunse in which I have found ova suspended in 

 the case of Halichondria are inhalant, but from the analogy 

 of Grantia labyrinthica we may assume that this is the 

 case. The adult ovum is about 0*067 mm. in total diameter. 

 Its outermost portion forms a distinct envelope around the 

 ovum proper. The envelope (fig. 2, env.) is fairly thick, and 

 projects at one side to form the peduncle (ped.), by means of 

 which the ovum is attached to the wall of the lacuna. The 

 substance of the envelope is but faintly granular, and stains 

 lightly; the peduncle in the specimen figured appears to be 

 hollow, but I should doubt if this is a constant character. 

 Within the envelope the ovum proper is suspended as in a 

 bag. It is spherical, about 0*053 mm. in diameter, uniformly 

 and rather coarsely granular, and stains deeply with borax 



