THE POSITION OF SPONGES, ETC. 457 



point ; they were well summed up by Schulze (1885, p. 191) 

 when he asserted "that the difference between the free 

 swimming ciliated larvae of sponges on the one hand, and 

 of Cnidaria on the other, is not more considerable than the 

 difference among sponge larvae themselves ". 



(4) The reversion of the layers at metamorphosis. This 

 fact, now sufficiently well established, is quite subversive of 

 the ccelenterate theory. It is only necessary to read the 

 arguments of the coelenterists to see at once how completely 

 it undermines their position. The coelenterate theory as- 

 sumes as the type or " Grundform " of sponges a hollow 

 sac-like or vase-like organism, with the body wall com- 

 posed of two layers, ectoderm and endoderm, and derived 

 from an embryo composed of the same two layers similarly 

 orientated. If, however, we make our comparison start 

 from the adult forms, then the larvae of sponges are quite 

 anomalous, in that they possess an ectoderm derived from 

 the macT-omeres, and often completely surrounded by 

 the endoderm. If, on the other hand, we lay stress on the 

 resemblances between the larvae any comparison of the 

 adult sponge to a coelenterate is out of the question. 



When, however, we try to decide by means of this point 

 whether sponges are or are not Metazoa, we do not get 

 much help, since in either case the interpretation of the 

 ancestral history presents much difficulty. If the larva, 

 say of a simple Ascon, be regarded as strictly homologous 

 with the very similar larva of a hydroid, it is very difficult 

 to understand why in the one case the component layers 

 should be reversed at fixation and in the other case not. If 

 on the other hand we regard the Ascon parenchymula 

 larva as a repetition in development of an ancestral stage 

 similar to Proterospongia, a free-swimming volvox-like 

 colony of Choanoflagellata in which certain zooids were 

 continually migrating into the interior, it is by no means 

 easy to form a clear idea of the steps by which such a form 

 came to reverse its layers and place its collared cells in the 

 interior. Hence the metamorphosis of the sponge larva 

 cannot be regarded as affording conclusive arguments in 

 favour of either the metazoan or the choanoflagellate 

 affinities of sponges. In so far, however, as it indicates 



