STRUCTQRE OF THE LARVA OF SPONGILLA LAOUSTRIS. 461 



flagellated chambers j and besides, it has bceu amply proved 

 that the ciliated cells of the larva are potential collar-cells. 



It may be argned, perhaps, that the occurrence of flagellated 

 chambers in the larva of Spongilla is a case of precocious 

 segregation. This term is a highly convenient one, and the 

 principle involved in it is often found to operate in nature ; but 

 one cannot help thinking that its assistance is invoked far too 

 often to solve the riddles presented to us by embryology. In 

 the case now under consideration the appearance of choauo- 

 cytes or collar-cells in the frcc-swimming larva of Spongilla 

 may be a case of reversion rather than of precocious segregation. 



However, the existence of choanocytes or collar-cells in the 

 sponges, as well as their early appearance in the free-swimming 

 larva, together with the ontogenetic method of differentiation 

 found in the Ascons, the most primitive of sponges, inclines me 

 to believe that sponges have been evolved independently from 

 Choanoflagellata. This conclusion is further strengthened by 

 the consideration that collar-cells do not occur in any of the 

 various phyla of the Metazoa. 



Conclusion. 



As has been stated already, this piece of work has been 

 carried out under the supervision of my teacher. Professor E. 

 R. Lankester, M.A., F.R.S., and Mr. E. A. Minchin, M.A. 

 To the former, whose pupil I have the honour of being, 1 wish 

 to offer my heartfelt thanks for the free use of his laboratory 

 and all its resources, as well as for many valuable hints and 

 suggestions kindly given, both in connection with the work done 

 and its publication. To the latter I am greatly indebted for 

 his most generous and invaluable assistance, especially in con- 

 nection with the technique, and am glad of the present oppor- 

 tunity of expressing my thankfulness. 



I have also to offer my sincerest thanks to the Principal and 

 Fellows of Jesus College, who not only continued my exhibi- 

 tion after I had completed the ordinary university course, but 



