160 SPONGES 



To obtain constructive evidence of a convincing nature in favour 

 of either of these views is a matter of extreme difficulty, if not im- 

 possible. On the side of the Choanoflagellate ancestry we may 

 urge the invariable presence of collar cells and their remarkable 

 resemblance to Choanoflagellata (see above, p. 53). The theory of 

 Enterozoic affinities of sponges, on the other hand, is based upon 

 their sexual reproduction and the resemblance of the early develop- 

 mental stages, culminating in a two-layered, planula-like larva, to 

 those of other Metazoa. The latter theory seems, therefore, at 

 first sight, to stand upon a wider basis than the former, but a closer 

 scrutiny leads to the conclusion that the supposed Enterozoic char- 

 acters of sponges are far from being of a very diagnostic nature. 

 In the first place, sexual reproduction by means of ova and sper- 

 matozoa is of widespread occurrence in plants as well as animals. 

 Secondly, the type of segmentation seen in any ovum depends, as 

 is well known, largely on its constitution, and in so far may admit 

 of explanation by purely physical laws. And finally, as regards 

 the germinal layers, the subsequent fate of these layers in the 

 sponge embryo makes it very difficult to homologise them with 

 those of other Metazoa. 



While, therefore, the characters that connect the sponges with 

 the Enterozoa are of rather a shadowy and vague nature, the pos- 

 session of collar cells stands out, at present, as a sharply defined and 

 very diagnostic feature in their organisation which links them to the 

 Choanoflagellata, and this view receives indirect support from the 

 many anomalies of sponge development which make it very diffi- 

 cult to bring them into line with other animals. 



If from the basis of a Choanoflagellate ancestry we try to reconstruct the 

 past history and evolution of the sponge phylum by the help of the stages 

 seen in embryology, we find the simplest condition typified in the larvae 

 of Ascons. The larva of Clathrina before immigration has commenced 

 may be regarded as a Protozoon colony composed of nutritive zooids, 

 together with a small number specialised for reproduction (cf. Figs. 

 57, 1, and 58, 1). Such colonies are seen in the Volvocineae and in 

 Proterospongia (Savile-Kent). In the sponge ancestor the nutritive 

 zooids were doubtless provided each with a collar and flagellum ; 

 in the sponge larva the flagellated cells serve during the free swimming 

 stage only for locomotion, and their nutritive function is in abeyance, 

 hence the collar is not developed on them until after the metamorphosis. 

 As time went on a third class of cell was developed by modification of the 

 nutritive zooids, as occurs in all sponge larvae. These new elements may 

 have had at first a digestive and distributive function, or they may have 

 been skeletogenous, or finally, they may have simply been a modified form 

 of flagellated cell, as exemplified in the larva of Oscarella, and specialised 

 perhaps for locomotion rather than nutrition. In any case the non- 

 reproductive zooids, at first all alike, became subdivided into a specially 

 nutritive set, retaining the primitive characters, ami another set specialised 



