'92 Mr. W. Marshall on the 



the Sponges, on the one hand, and the Cnidaria, on the other, 

 is, on the whole, not more important than that between the 

 various Sponge-larvse among themselves. But as the funda- 

 mental differences in the structure of the two groups only- 

 showed themselves after the metamorphosis, we are justified 

 in the assumption that the divergence of the two lines did not 

 begin before that phylogenetic developmental stage which 

 represents the ciliated larva on the point of metamorphosis. 



Sollas takes quite a different standpoint, as strikes one at 

 once on reading the proposition that he places at the head of 

 his argument, namely, that it is difficult to suppose that such 

 complicated structures as the collared flagellate cells of the 

 Sponges could so closely resemble the Choanoflagellata, and 

 at the same time be of independent origin. He explains the 

 metazoic character of the Sponges, using Lankester's term, as 

 " homoplastic," and their Infusorian character as phylogenetic, 

 i. e. he believes the latter to be inherited, the former newly 

 acquired. He proceeds : — Only two characters of the Sponges 

 are essentially of metazoic nature, namely, the presence of both 

 kinds of sexual reproductive bodies and of a gastrula. As 

 regards the former, we find also in plants two kinds of sexual 

 products, and what plants and animals could have formed inde- 

 pendently of each other Sponges and Coelenterata might also 

 have acquired independently. In opposition to the second 

 character, the formation of the gastrula, it is to be remarked, 

 in the first place, that the flagellate cells of the habit of the 

 Choanoflagellata occur very early in the ontogeny of the 

 Sponges, at least before the formation of this gastrula, as is 

 clearly the case in the araphiblastula of Sycon raphanus. 

 Secondly folding, and therefore the formation of a gastrula, is 

 one of the most frequent of all processes in the different develop- 

 mental processes of animals, and is probably easily explicable 

 by a simple mechanical process. So much, at least, is certain, 

 that foldings in numerous cases originate quite similarly and 

 independently of inheritance, and may lead to the foundation 

 of organs which may indeed be " homoplastic," but certainly 

 not homologous. Further, we see that the formation of a 

 gastrula in Sponges, as also in Cnidaria, may take place in 

 two ways, namely, by invagination and by cleavage of the 

 mesenchyma, and one or other of these two modes at least 

 cannot be explained by inheritance. Once more asserting 

 the early occurrence of the cells like Choanoflagellata in the 

 sponge-larva3, Sollas comes to the conclusion that the Sponges 

 may have developed themselves independently as a special 

 phylum from the Choanoflagellata, and he proposes to separate 

 them from the Mefazoa under the name of " Parazoa." 



