212 ARTHUR DENDY. 



do not think that Bidder has by any means proved that the 

 '' pendent cell body^^ belongs to the ectoderm at all. It seems 

 quite as reasonable to believe that it is the presence of 

 subdermal gland-cells, in many cases at any rate, which has 

 led Bidder to believe in the general occurrence of flask-shaped 

 or onion-shaped glandular ectoderm-cells. Until we have 

 more light on the subject I prefer to retain my old belief in 

 subdermal gland- cells, but I am quite open to conviction on 

 this point, which, with the material at my disposal, I am 

 incapable of deciding for myself. 



The observations of Minchin (25, 26) are also very note- 

 worthy in this connection. This author explains the flask- 

 shaped condition as due simply to contraction of the normal, 

 flattened, plate-like cells, and his observations upon Leu- 

 cosolenia certainly support this view. Subdermal gland- 

 cells do not appear to occur, at any rate as a rule, in the 

 Homoccela, and so far as these sponges are concerned I am 

 inclined to accept Minchin's explanation. 



I must now describe what few additional observations I have 

 myself been able to make on the ectoderm of the Heterocoela. 

 These observations support the view that the ectoderm is 

 composed of a single layer of flattened plate-like cells, which 

 retain to a greater or less extent the power of contraction. I 

 believe that the character of this epithelium is, usually at any 

 rate, the same throughout, from osculum to prosopyles, although 

 it is much more difficult to detect on the exposed outer 

 surface of a dermal cortex than on the protected surface of an 

 inhalant canal. I have observed it most satisfactorily in a 

 specimen of Vosmaeropsis Wilsoni (4) which I took the 

 precaution to kill with osmic acid. This sponge is provided 

 with a very distinct oscular sphincter, which has much the 

 same structure as that described by Minchin inLeucosolenia 

 clathrus (25). It is composed of two layers of epithelial cells 

 with probably a very thin layer of gelatinous mesoderm (ground 

 substance) between them. Both layers of cells are regarded 

 by Minchin inLeucosolenia as ectodermal, and (as a mere 

 matter of convenience, for we must draw the line between 



