STUDIES ON THE COMPARATIVE ANATOMY OF SPeNGES. 29 



Chalinissa communis than they do with those of the closely 

 related Grantia compressa. We must associate with this 

 fact the fact that the arrangement of the pores in Grantia 

 labyrinthica also agrees much more closely with that which 

 usually obtains in the Chaliniuse than with that found in 

 Grantia compressa, thus affording another instance of 

 homoplasy. 



The cells which Sollas (13) describes and figures as possible 

 sense-cells {'' sesthocytes ") in the Tetractinellida appear to be 

 of a more problematical character. 



Reproductive Cells. — The only reproductive cells which 

 I have met with are the ova. These, as in other sponges, are 

 obviously derived from amoeboid cells, and in the earliest 

 stages of their development it is impossible to distinguish 

 between the two. Later, however, the ova become more 

 rounded off, and the nucleus becomes large and distinct. 



It is now generally admitted that the ova of sponges are 

 fertilised by spermatozoa probably of other sponges, which 

 gain admission to the sponge with the inflowing current of 

 water. No one, however, so far as I can discover, has attempted 

 to find out whereabouts in the sponge the union of ovum and 

 spermatozoon takes place. Having followed the spermatozoon 

 into the canal system they leave it there to take care of itself, 

 forgetting that, unless some special arrangement exists to 

 prevent such a catastrophe, it will speedily be washed out 

 again through the osculum without ever having had a chance 

 of fulfilling its errand. The general assumption would seem 

 to be that the spermatozoon loses its way through the walls of 

 the canals, and wanders about in the gelatinous mesoderm 

 until it happens to come across an ovum. It seems highly 

 improbable that this should be the case, for it would be a 

 strange thing if the spermatozoa bored their way through the 

 epithelium, as they would have to do in order to get into the 

 gelatinous ground-substance, without any obvious inducement 

 to do so. Mr. Carter seems to have come nearer to the truth 

 than anyone, but without realising the true significance of 

 what he saw. He says (24) that the ovum in Grantia com- 



