614 W. J. SOLLAS. 



independent modification, due, no doubt, to the similar action 

 of physical causes. If the original gastrula was formed by 

 fission then those secondarily produced by invagination are 

 independent modifications. But if secondary gastrula, having 

 considerable similarity to each other, have thus been produced 

 independently from a primitive gastrula, it becomes quite 

 possible that the primitive gastrulse themselves are independent 

 modifications of groups of cells originally possessing very 

 difierent characters. It is also not to be forgotten that the 

 divergencies between the development of Sponges and Coelen- 

 terates are almost as striking as the resemblances. It may be 

 objected to this line of argument that in the Sponges originally 

 similar cells have become differentiated into muscle fibres and 

 connective-tissue fibres which are homoplastic but not homo- 

 logous with those of the true Metazoa, and hence that inde- 

 pendent origin of similar cells is just as probable as the inde- 

 pendent origin of similar organs. Admitting this, one would point 

 out that the choano-flagellate cells of the Sponge appear early in 

 its embryological history and persist as its special characteristic, 

 while the muscle and connective-fibre cells appear compara- 

 tively late in life, and are produced by the Sponge to meet 

 special exigencies. 



It will appear, therefore, according to the view here advo- 

 cated, that while the Sponges are evidently homoplastic with 

 the Metazoa they are of independent origin, having arisen as a 

 separate phylum from the choano-flagellate Infusoria. For 

 this phylum I propose the name Parazoa ; the relations of the 

 three groups into which the animal kingdom will then be 

 divided are expressed in the diagram below. 



/METAZOA 



Parazoa 

 \ (Sponges) 



PROTOZOA 



The gastrula of the Parazoa is essentially distinguished from 



