456 SCIENCE PROGRESS. 



or less retracted, often completely so, when the sponge is 

 contracted and closed up. Hence, for my part, I should not 

 expect to find the ciliated cells of the larva provided with 

 collars, even if it were quite certain that the theory of choano- 

 flagellate descent were the true one. But on the theory of 

 the continuity of the ciliated cells, which we have already dis- 

 cussed, the ciliated cells of the larva pass directly into the 

 collar cells of the adult and develop collars after the metamor- 

 phosis, when the sponge begins to feed and grow. Hence it 

 follows that the ciliated cells of the larva do possess the power 

 of developing collars, and might be regarded as potential 

 collar cells, in which the function of nutrition is temporarily 

 in abeyance, and as a result the collars are not protruded. 



From the foregoing review of sponge characteristics we 

 may pick out four points of primary importance in the dis- 

 cussion of their affinities : — 



(i) The unfailing possession of collar cells, and their 

 great resemblance to Choanoflagellata. This, as we have 

 seen, is the feature which has been the mainstay of those 

 who advocate the relationship of sponges to Choanoflagellata 

 or their descent from them, while those who advocate a 

 contrary view are obliged to minimise the resemblances, or 

 declare them due to analogous adaptation. 



(2) The reproduction by ova and spermatozoa. This, 

 on the other hand, is a feature which has furnished support 

 to those who regard sponges as Metazoa of some sort or 

 other. In view, however, of the many analogous sexual 

 processes known to occur amongst Protozoa and in plants, 

 it is perhaps not safe to lay too great stress upon this point, 

 especially as the details of the process seem to be of a type 

 rather more primitive than in other Metazoa. 



(3) The formation of two germ layers by processes not 

 specially characteristic of sponges, and the possession of a 

 larva very similar to the coelenterate planula. This is 

 certainly the strongest argument for the Metazoan affinities 

 of sponges, and has always been much employed by the 

 supporters of this view, especially by the coelenterists. We 

 have already quoted many of the arguments based on this 



