458 SCIENCE PROGRESS. 



the ciliated cells of the larva to be potentially collared cells, 

 as pointed out above, the reversion of the layers is slightly 

 in favour of the choanoflagellate theory. 



It is very evident that we have at present but two 

 theories to choose between ; either to regard sponges, with 

 Biitschli, Sollas, and Delage, as descended from choano- 

 flagellate ancestors independently of the Metazoa ; or to 

 regard them with Maas and, apparently Balfour, as true 

 Metazoa composed of the two primary germ layers, ecto- 

 derm and endoderm, which have become reversed in 

 position in the adults. The choice between these two 

 theories will depend upon which of two assumptions we 

 find most difficult. If sponges are Metazoa, the collar 

 cell, occurring in no other Metazoa, must have been in- 

 dependently acquired. If sponges are descended from 

 Choanofiagellata, then the sexual reproduction, the seg- 

 mentation of the ovum, and the formation of the two 

 germ-layers, must be processes analogous, and ndt homol- 

 ogous, with the similar processes in Metazoa. For my part 

 I must confess that the second assumption seems to me at 

 present the^hardest to swallow. The theory of the meta- 

 zoan nature of sponges offers in the present state of our 

 knowledge fewer difficulties than the theory of independent 

 descent from the Choanofiagellata. Whether or not a 

 closer acquaintance with the structure and development of 

 sponges will confirm this view remains to be seen. How 

 cautious we should be in pinning our taith to either one of 

 these positive views is sufficiently shown by the history 

 and present position of the coelenterate theory. 



LIST OF WORKS CITED IN THE TEXT. 



DujARDiN, F. (184 1.) Histoire Naturelle ties Zoophytes. In- 



fusoires. Paris, 1841. 

 Carter, H. J. (1848.) Notes on the Species, Structure, and 



Animal ity of the Fresh Water Sponges. Ann. Mag. Nat. 



Hist. (2), i., pp. 303-311, 1848. 

 Perty, M. (1852.) Ziir Kenntniss icieinster Lelmisformen in 



der Schweiz. Bern, 1852. 

 Leuckart. R. (1854.) Bericht iiber die Leistungen in der 



Naturgeschichte der niederen Thiere wahrend der Jahre, 



1848-1853. ArcJi. f. Naturgesch., vol. xx., pp. 289-473. 



