PROF. DELAGE ON SPONGES. 6 1 



directions before selecting one ; which indicates that some trifling 

 (although constant) cause can decide it to choose the way which it 

 finally does adopt. 



In Sycandra, one of the calcareous Sponges, before the larva is 

 hatched, it is known that it is invaginated in the normal way, 

 during a transitory stage named psendogastrula. In the common 

 theory, this stage is rather puzzling and no satisfactory explanation 

 has been given of it. In my theory, it is quite clear, being only an 

 atavistic reproduction of the normal invagination. 



So, we may characterize the Spongida in the following manner : 

 ivhat in the larva of the Spongida has the histological character's of 

 an ectoderm, vis. the flagellated cells, is really the ectoderm ; tvJiat has 

 the characters of an endoderm, vis. the granulated cells, is really the 

 endoderm ; bnt, the invagination is reversed, the ectoderm being in- 

 vaginated into the endoderm to form the choanocytes, i.e. the digestive 

 epithelium, while the endoderm remains {Calcarea) or passes {Silicea) 

 outside to form the skin. And I say for the last time : is not that a 

 character more important than is strictly necessary to mark off 

 the Spongida as a distinct phylum among the Metazoa, sharply 

 distinguished not only from that of the Coelenterata, but also from 

 any othpr in the animal kingdom ? 



Now if, trying to advance more deeply into the subject, we turn 

 to the phylogenetic point of view, we must put the question : When 

 have the Spongida parted from the Metazoa ? 



I am of the opinion that we have no means to solve such 

 questions with certainty, and that imagination plays by far too 

 large a part in our attempts to establish the genealogical tree. 

 But if we refer to the only test of the matter we can have, that is, 

 to the embryological data, we have some right to suppose that 

 the Spongida did keep up with the other Metazoa only during 

 the few stages corresponding to the ontogenetic evolution from 

 the Q^% to the blastula and have parted free before the gastrula 

 stage. 



The Spongida are a main, although small, branch arising directly 

 from the stem of the genealogical tree, independently of the branches 

 of the Coelenterata and of the other Metazoa. 



The discussion was continued by Mr E. A. MiNCHIN, who laid 

 on the table some letters of Dr Dobie, substantiating his claim to 

 share with Dujardin the honour of having first recognized the 

 animal nature of Sponges ; the following is an abstract of Mr 

 Minchin's remarks. 



I. Historical. There is no group of organisms whose sys- 

 tematic position is so much disputed, at all periods as well as at 

 the present day. 



Plants or Animals f Up to the end of the first half of the 

 19th century, it was still a matter of dispute whether sponges 



