612 W. J. SOLLAS. 



Classificatory Position of the Sponges. — The early 

 appearance of flagellate^ and^ indeed, choano-flagellate cells in 

 many, if not all. Sponge embryos, and their persistence through- 

 out life as the characteristic cells of the adult Sponge, is a very 

 remarkable phenomenon, and on the theory of heredity it meets 

 with its most plausible explanation by supposing (as Saville 

 Kent does) that the Sponges are directly descended from the 

 choano-flagellate Infusoria. It is difficult to suppose that such 

 complicated structures as the choano-flagellate cells of the 

 sponge should so closely resemble the choano-flagellate Infu- 

 soria and yet be of independent origin. 



On the other hand, the Sponges are quite clearly not 

 Protozoa; the individuality of the component cells of the 

 Sponge is subordinated to that of the total organism to an 

 extent and in a manner which meet with no parallel amongst 

 the Protozoa J in the lowest Sponges a definite epithelial layer, 

 together with a mesoblastic connective jelly, always coexists 

 with the flagellate cells, while in the higher Sponges certain 

 cells are diflFerentiated into muscle-fibre-cells which are remark- 

 ably similar to the involuntary muscle-cells of the higher 

 animals, and which are arranged in definite muscular sphinc- 

 ters ; at the same time other cells are difi'erently modified to 

 form a tissue which curiously simulates fibrous connective 

 tissue. Indeed, so far as tissue differentiation is concerned, a 

 geodine Sponge is, on the whole, a more highly organised 

 animal than a fresh water Hydra. In the reproduction of 

 the sponge, even in such forms as Halisarca, by ova and sper- 

 matozoa we meet with another metazoan character. 



It makes no difference if some Sponges develop partheno- 

 genetically,^ of which at present we have no proof, since 

 parthenogenesis frequently occurs among the higher Metazoa. 

 No argument is to be drawn from the segmentation of the 

 ovum, since very regular segmentation occurs in the product of 

 conjugation of some. Protozoa; but the arrangement of the 

 resulting cells in two layers, and the formation of a gastrula, 



1 I expect that parthenogenesis will be foimd to be of much more frequent 

 occurrence among the lower Metazoa than is at present suspected. 



