62 SECOND GENERAL MEETING. 



were plants or animals ; this controversy was laid to rest by the 

 discovery of cilia by Dujardin (1841) and Dobie (1850), as well as 

 by subsequent researches of Lieberklihn and Carter. The animal 

 nature of sponges was thus established, but their position was still 

 uncertain. 



(a) They were regarded by earlier writers as Protozoa, especially 

 after the discovery by James Clark of the collar cells and their 

 great resemblance to Choanoflagellata. 



The Protozoan theory was soon, however, given up, since it is 

 irreconcileable with any rational definition of Protozoa ; Sponges 

 are composed of tissues, which are made up of differentiated 

 cells. 



(d) They were compared by Leuckart (1854) to polyps without 

 tentacles, and hence regarded as true coelenterates ; the same idea 

 was expressed by Miklucho-Maclay and strongly advocated by 

 Haeckel, who extended the comparison not only to the adult 

 organisms but also to the larvae. In both groups, it was supposed, 

 there is a two layered larva, composed of " exoderm " and " ento- 

 derm," from which two layers arose, the dermal and gastral layers, 

 respectively, of the adult. 



The Ccelenterate theory became dominant, with the modification 

 of Schulze, who distinguished between the epithelial and the skele- 

 togenous portions of the dermal layer and hence made out three 

 layers in the body wall, ectoderm, mesoderm and endoderm. This 

 theory, however, found dissentients ; two other views were advo- 

 cated. 



(c) That Sponges were Metazoa but not Ccelenterates; opinions 

 differed as to the question of the homology of the germ la}'ers. 

 Balfour, Gotte, Vosmaer, Maas. 



(d) That Sponges were not even Metazoa, but an independent 

 phylum descended separately from Protozoa ; Butschli, Sollas, 

 Delage. 



Higher animals Higher animals 



/ I . Coel. 



Sponges . / Coel. Sponges 



Prot. 

 (^) 



II. Present Position of Question lies between b, c and d. 



Are Sponges Ccelenterates.^ If not, what are they? There 

 are two methods of attacking the question. 



1st. The Comparative Anatomy of the Adult forms. 



2nd. The Embryology and especially the all important question 

 of the germ layers and the share taken by them in building up 

 adult. 



