POSITION OF THE SPONGIDA. 667 



of an osculum and pores. The ciliated collared cells which line 

 the ciliated chambers, or in some cases the radial tubes, are 

 undoubtedly derived from the invaginated cells, and if there is 

 any truth in the above suggestion, the collared cells in the adult 

 Sponge must be mainly respiratory and not digestive in function, 

 while the normal epithelial cells which cover the surface of the 

 sponge, and in most cases line the greater part of the passages 

 through its substance, must carry on the digestion 1 . If the 

 reverse is the case the whole theory falls to the ground. It has 

 not, so far as I know, been definitely made out where the 

 digestion is carried on. Lieberkuhn would appear to hold the 

 view that the amoeboid lining cells of the passages are mainly 

 concerned with digestion, while Carter holds that digestion is 

 carried on by the collared cells of the ciliated chambers. 



If it is eventually proved by actual experiments on the nutri- 

 tion of Sponges, that digestion is carried on by the general cells 

 lining the passages, and not by the ciliated cells, it is clear that 

 neither the ectoderm nor entoderm of Sponges will correspond 

 with the similarly named layers in the Ccelenterata and the 

 Metozoa. The invaginated entoderm will be the respiratory layer 

 and the ectoderm the digestive and sensory layer ; the sensory 

 function being probably mainly localised in the epithelium on 

 the surface, and the digestive one in the epithelium lining the 

 passages. Such a fundamental difference in the germinal layers 

 between the Spongida and the other Metazoa, would necessarily 

 involve the creation of a special division of the Metazoa for the 

 reception of the former group. 



1 That the flat cells which line the greater part of the passages of most Sponges 

 are really derived from ectodcnnic invaginations appears to me clearly proved by 

 Schulze's and Rarrois' observations on the young fixed stages of Halisarca. Ganin 

 appears, however, to maintain a contrary view for Spongilla. 



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