438 SCIENCE PROGRESS. 



whose views we have been discussing, though the theor)^ 

 of a separate choanoflagellate ancestry for a class of coelen- 

 terates reminds one strongly of Kent's admission of a Pro- 

 tascus ancestor for both corals and a class of Protozoa. 

 Doubtless, however, this admission on the part of Lenden- 

 feld was also meant to carry with it the vaguest possible 

 significance and simply as an acknowledgment of his faith 

 in the doctrine of evolution. 



From the above exposition of the various theories that 

 have been held as to the nature of sponges, it will be seen 

 that it would be difficult to brings forward a new and orioinal 

 opinion on this subject. No one has yet sought to prove 

 that sponges are degenerate molluscs, arthropods or worms, 

 or that they are the progenitors of Vertebrates, and there 

 is undoubtedly an opening for a physiologist to find the 

 ancestor of sponges in the king crab. Also the theory 

 of Goette that sponges are made up of endoderm alone, 

 suggests a counter hypothesis that they are composed of 

 ectoderm alone. When these two classes of speculations 

 have been brought forward there will remain, so far as I 

 can see, absolutely no new theory of sponge affinities to be 

 propounded. 



I propose now to discuss shortly the leading character- 

 istics of sponges and the evidence they afford in favour of, 

 or against, one or other of the above theories. Even if it 

 be not possible at present to arrive at a conclusion as to the 

 true nature of sponges, it may be at least possible to de- 

 molish and eliminate some of the numerous theories that 

 have been put forward, and so narrow the field of inquiry. 

 And at the outset it must be repeated that the Protozoan 

 nature of sponges cannot now-a-days be seriously discussed. 

 We have to consider whether sponges have, or have not, 

 a common descent with the Metazoa, and if this question 

 be answered in the affirmative, then whether and to what 

 degree they are or are not coelenterates. Perhaps the 

 most striking feature of sponges as compared with other 

 animals, is to be found in their peculiar "collar cells". In 

 the lowest sponges, the Ascons, the collar cells form a con- 



