262 EDWARD A. MINCHIN. 
Ascetta clathrus, Ascaltis cerebrum, &c. Haeckel 
discusses it at some length, and comes to the conclusion— 
rightly, I think—that it is “durch Anpassung erworben ” 
(‘ Kalkschwimme,’ Bd. i, pp. 209, 270). He also brings into 
the discussion the sieve-plate of Huplectella speciosa. I 
think it is not without interest to find that, in a form most 
commonly lipostomous, the oscula when present are partially 
closed by a sieve membrane. Were the opening in this mem- 
brane to close up, and the inner layer of cells to become col- 
lared cells like the remaining endoderm, lipostomy would be 
at once attained. The osculum in fig. 3 might be an osculum 
thus closed; but I believe not, on account of its extremely 
small size. 
In its biological or physiological aspect, I believe this mem- 
brane is of use for keeping intruding auimals out of the gas- 
tral cavity. We can distinguish three ways in which sponges 
do this. One way is by a fringe of projecting spicules round 
the osculum, which effectually prevents the ingress of animals 
into the gastral cavity, though not their exit; e.g. Sycon 
ciliatum. Other sponges have an iris-like membrane over 
the osculum, which is sensitive, and can be closed by a 
sphincter (see Haeckel, ‘ Kalkschwamme,’ Bd. i, p. 266). A 
third method is by a sieve-plate, as here and in Euplectella, 
&c. Whether the sieves of Euplectella and of my Ascetta 
are homologous or not, I have little doubt they are ana- 
logous. 
On the remaining histology of this sponge I have a few 
observations to record, though they are not as complete as I 
could wish. First of all as to the— 
Endoderm.—lIn sections of the sponge hardened in osmic 
immediately after plucking it from its native hauuts, the collar- 
cells present the appearance of a columnar epithelium com- 
posed of cells quite distinct from one another, each with a 
rounded slightly enlarged base stuck into the jelly and con- 
taining the nucleus, while the upper part of the cell is pro- 
longed into a collar about half or two thirds the height of the 
cell. In the centre of the collar the flagellum is usually 
