454 - SCIENCE PROGRESS, 



external layer of the larva becomes the internally placed 

 collared epithelium of the adult, while from the inner mass 

 of the larva arise the remaining tissues of the sponge body. 

 There still remain two important cases where statements to 

 the opposite effect remain on record uncontradicted, namely, 

 the cases oi Halisarca and Plakina. In view, however, of 

 the great similarity of their larvae to those of the lower 

 Ascons it is difficult to believe their development to be of 

 so fundamentally different a type as the recorded observa- 

 tions would, force one to suppose. In the present condition 

 of our knowledge it seems to me, that wherever statements 

 exist to the effect that the inner and outer layers of the larva 

 pass respectively into the inner and outer layers of the 

 adult, we are justified in regarding such statements as false 

 until they are proved to be true, instead of believing them 

 to be true until they are proved to be false. 



In order to complete and conclude our review of the 

 main features of sponge embryology, there remains one 

 small but important point for discussion ; namely, Do the 

 collar cells so characteristic of the adult ever occur in the 

 larva.'* The point is one which has been thought to be of 

 considerable importance as bearing upon the question of 

 the affinities of sponges with Choanoflagellata. Kent, who 

 as we know was a firm believer in a close relationship 

 between the two groups, described collar cells with large 

 and distinct collars as occurring in numerous larvae, and 

 tried to prove that the sponge larva was nothing more than 

 a "colonial aggregation of typical collared zooids " (1881, 

 p. 183). He also quoted Barrois (1876, pi. xv., fig. 29) as 

 having figured similar collar cells in sponge larvae. 



Schulze (1885) on the other hand set himself to dis- 

 prove Kent's statements, while arguing (p. 189) that on the 

 theory of a descent of sponges from Choanoflagellata it 

 might be expected that collar cells would make their 

 appearance in that phase of sponge development which 

 corresponds to a Protozoan colony in phylogeny, namely, 

 in the blastula stage. Schulze, however, searched in vain 

 for collars to the ciliated cells of Sycandra, and declares 

 himself convinced (p. 182) that Kent's figures of "swarm- 



