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 
