CHAP. Ill 



PORIFERA 



119 



s.m. 



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

 kcrp up an outwardly flowing current of water through the osculum . 

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

 name is the presence of a thin 

 soft membrane of protoplasm 

 the collar which projects from 

 tin- margin of the free end of the 

 cell, forming a kind of tube or 

 funnel, 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 

 < ell hocly. The collar can be re- 

 tracted completely into the cell- 

 body and as a consequence may 

 invisible in sections not pre- 



ired very carefully. The choano- 

 :ytes line the whole cavity of the 

 sponge except a region in the 

 leighbourhood of the osculum 

 which is floored in by the sieve- 



lembrane a thin perforated mem- 

 >rane stretching straight across the 



ivity of the sponge some little 

 listance internal to the osculum 

 (Fig. 54, s.m). 



The dermal layer consists of a 



itrix of clear jelly, resembling 

 the mesogloea of the Coelenterates, 

 rith which are associated several 

 types of cells. Covering the whole 



eternal surface and extending 



iwards at the osculum as far as 



ic sieve-membrane is the dermal 

 jpithelium (Fig. 55, d.e) a layer 



)f closely fitting polygonal cells so flattened out as to appear in 

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



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

 the genus Leucosolenia. these cells are highly contractile and 



A* 1 



FIG. 54. 



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

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



