ELISHA MITCHELL SCIENTIFIC SOCIETY. 47 



this view, does not represent a primitive organ (Urmiind), 

 but merely comes into existence owing to the special, and 

 highlv modified, method of forming the entoderm. We 

 do not, therefore, have to construe the Oscarella develop- 

 ment (with Heider and SoUas) as meaning that a gastrula 

 ancestor settled mouth downward, and that the mouth 

 gradually became functionless, finally closing up, while a 

 new series of openings, pores and osculum, were estab- 

 lished. 



The only remaining point I wnsh to speak of is the rela- 

 tion of the sponges to the Coelenterates. That the two 

 groups have had a common ancestor in the Parenchymella 

 is highly probable, but the similarity between the Olynthus 

 and the simplest Coelenterates inclines one to go further, 

 and at any rate homologize the paragastric cavity of the 

 former with the gastric cavity of the latter. This, of 

 course, is done by authors like Sollas, who derive both 

 groups from a common gastrula ancestor. Whether the 

 osculum of the Olynthus is also homologous with the gas- 

 trula mouth, as Haeckel originally held, is a question 

 which needs for its answer more facts relating to the actual 

 use to which the osculum is put in the simplest sponges. 

 Sollas and Heider urge against the homology the fact that 

 the Coelenterate larva attaches by the pole opposite the 

 blastopore, while in the sponge larva the blastopore is at 

 the pole of attachment. But this I cannot regard as a 

 very strong argument, for (with Metschnikoff) I do not be- 

 lieve that the opening into the gastrula cavity represents a 

 primitive organ (mouth of an ancestor). And if it does 

 not, but is merely an incidental product of a particular 

 mode of entoderm-formation employed by the animal, it 

 has no bearing on the question of homology between oscu- 

 lum and mouth. Consequently the fact that in the attach- 

 ing coelenterate and sponge larvae the blastopore is at oppo- 



