No. 3-] DEVELOPMENT OF MARINE SPONGES. 363 



the immigration of the entoderm. The blastopore of the 

 sponge gastrula on this view does not represent a primitive 

 organ (Urmund), but merely comes into existence owing to 

 the highly modified method of forming the entoderm. We do 

 not, therefore, have to construe the Oscarella development (with 

 Heider and Sollas) as meaning that a gastraea ancestor settled 

 mouth downwards, and that the mouth gradually became 

 functionless, finally closing up, while a new series of openings, 

 pores and oscula, was established. 



The only remaining point I wish to speak of is the relation 

 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 gastrula-like ancestor. 

 Whether the osculum of the Olynthus is also homologous with 

 the coelenterate 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 coelen- 

 terate 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 I do 

 not believe 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 

 endoderm-formation, it becomes evident that the position of 

 the blastopore at opposite poles in sponge and coelenterate 

 larvae has no bearing on the question of homology between 

 mouth and osculum. 



It is, moreover, doubtful if any such sweeping distinction 

 can be drawn between the larvae of the two groups, for it is a 

 question whether any sponge larva has a particular pole by 

 which it must attach. Even in Sycandra, Schulze records 

 (25, p. 274) that exceptional cases occur which cannot be 

 regarded as pathological, in which fixation takes place not by 



