460 RICHARD EVANS. 



hypoblast are formed, and then the reproductive cells become 

 separated from one of these along witli the mesoblast where 

 this latter occurs. Again, we may invoke the aid of the 

 Protozoa, and notably of Volvox, a colonial Protozoon, in which 

 a phenomenon occurs which is on a par with what takes place 

 in sponges. In Volvox the reproductive cells become marked 

 out from all the others as male and female cells, but the vege- 

 tative cells remain all alike. Clearly the Volvox colony with 

 its reproductive cells is comparable with the larvae of calcareous 

 sponges at a time when they consist of only a few posterior 

 granular cells and of ciliated cells. 



In Volvox no further division of labour takes place among 

 the ciliated or vegetative cells of the colony, but it would not 

 be very difficult to imagine the ciliated individual cells becom- 

 ing differentiated to two groups in the same way as those of 

 the larvae of Leucosolenia and Sycandra. I conclude, therefore, 

 that the order of differentiation of the sponge cells from the 

 phylogenetic point of view, as well as from the ontogenetic — in 

 the Ascons at least — is, first, into reproductive and locomotor 

 cells ; and secondly, the latter become differentiated into two 

 groups, one without flagella and collars, and another which 

 retains these cell organs. 



These considerations seem to cast a doubt upon the whole 

 idea of germ layers in the sponge larva. Given a group of 

 cells such as is found in the sponge embryo at the close of 

 segmentation, two layers, an outer and an inner, are bound to 

 appear sooner or later. Such a thing might easily happen in 

 the case of Volvox, in the same way as it does in the case of 

 the sponge larvae. 



Schulze's (13) great argument against the theory of the 

 independent origin of sponges from Choanoflagellata is that 

 they should have as a necessary consequence the occurrence 

 of choanocytes in the larva. Though I do not agree with 

 Schulze, rather than controvert the statement I am prepared 

 to meet his demand. I venture to point out that such cells do 

 often occur in the larva of Spongilla. They do not occur on 

 the outside, it is true, but in the inner mass in the form of 



