CHAP. Ill 



PORIFERA 



119 



the centre of the cavity of the sponge. These flagella beat actively and 

 keep up an outwardly flowing current of water through the osculum 

 The most peculiar feature of the choanocyte and that which gives it its 

 name is the presence of a thin 



soft membrane of protoplasm — os. 



the collar — which projects from 

 the margin of the free end of the 

 cell, forming a kind of tube or 

 funnelj of which the axis is occupied 

 by the flagellum. The protoplasm 

 of the collar shows active streaming 

 movements by which food-particles 

 coming against and adhering to the 

 collar are carried down into the 

 cell-body. The collar can be re- 

 tracted completely into the cell- 

 body and as a consequence may 

 be invisible in sections not pre- 

 pared very carefully. The choand- 

 cytes line the whole cavity of the 

 sponge except a region in the 

 neighbourhood of the osculum 

 which is floored in by the sieve- 

 membrane — a thin perforated mem- 

 brane stretching straight across the 

 cavity of the sponge some little 

 distance internal to the osculum 

 (Fig. 54, s.m). 



The dermal layer consists of a 

 matrix of clear jelly, resembling 

 the mesogloea of the Coelenterates, 

 with which are associated several 

 types of cells. Covering the whole 

 external surface and extending 

 inwards at the osculum as far as 

 the sieve-membrane is the dermal 

 epithelium (Fig. 55, d.e) — a layer 

 of closely fitting polygonal cells so flattened out as to appear in 

 section merely as a fine line with a dot here and there in its course 

 representing a nucleus. In some of the allied sponges, though not 

 in the genus LeucosoleniU; these cells are highly contractile and 



Fio. 54. 



A young Ascon sponge, x 60. os, Osculum ; 

 p, pore ; s.m, sieve-membrane ; sp, spicule. 



