138 Ectoderm and Spongollasts in Keratosa. [June 16, 



call a Metschnikoff cell or not. I consider the natural form of active 

 collar cells, in this as in other sponges, as being more separated in 

 their upper parts than in his figures, a large part of the surface of 

 the cell being washed by the water ; this area behind the collar I 

 have always regarded as the ingestive surface. Mr. Minchin believes 

 that he has obtained evidence that the ectoderm cells really push 

 their way to the endoderm ; holding the views I do as to excretion, I 

 shall be very glad if re-investigation causes me to admit this correc- 

 tion. 



Apart from our common ground, I have made the indigo-carmine 

 experiments with which the Society has been acquainted, and Minchin 

 has made his valuable observations on the changes in form of the 

 epithelium surrounding the oscula of Ascetta clathrus* Whatever 

 weight should be attached to these two factors in deciding the 

 meaning and function of the flask-shaped epithelium, I wish to 

 establish the undoubted presence of such a structure in Cacospongia, 

 and presumably in all Porifera. Examining Cacospongia, and com- 

 paring Merejkovsky's figure of Halisarca, any one will be convinced 

 that this is the true explanation of the puzzling appearances seen 

 by various authors on the outer surface of other sponges I would 

 take as examples, among very many, Schulze on Halisarca Dujardini 

 and Chondrosia reniformis, Polejaeff on JantTiella, Sollas on Stelletta 

 Normani and Isops Phlegrcei, Vosmaer on Tentorium (TJiecophora) and 

 the larva of Myxilla. Yosmaer's figure of Myxilla (fig. 8, PL XIV), 

 unites with Metschnikoff's of the young Halisarca, to show that 

 the flask, shaped cells appear at the earliest stages, and are probably 

 formed by direct metamorphosis from the columnar epithelium of the 

 free larva. On the other hand, the direct observations of flat epithe- 

 lium on the outer surface are strikingly few.f 



I hold that it may take rank as an established fact that in all 

 groups of sponges the flask-sha,ped epithelium does occur. It must 

 be recognised that the structure we are to expect to find on the 

 exterior of any sponge is a zone of hyaline tissue traversed by necks 

 which unite the centre of each silver area to the nucleus and cell- 

 body pertaining to it. This, whatever be its physiological explana- 

 tion, is the structure most commonly to be met with, and not a flat 

 epithelium, such as clothes the canals. 



And I believe that these cells are excretory, and identical with the 

 spongoblasts. 



* I should say that I can support the figure he gives as being an accurate repre- 

 sentation of transitions in form of epithelial cells to be met with in all the Calcarea. 

 I have hitherto ascribed to them an entirely different significance that where the 

 wall is thin, the special elongated flask-shaped form of cell is not required for 

 excretion. 



t The descriptions of flat epithelium generally flagellate by E. v. Lendenfeld 

 an; numerous and detailed. I attach no credit to them. 



