40 



ZOOLOGY 



cells, which line the central cavity and the ducts opening into 

 it ; and (ii.) collared cells, which line the flagellate chambers, and 

 in the Homocoela the general cavity. In Grantia the flagellate 

 chambers do not open directly into the central cavity, but into 

 a short exhalent canal (2, Fig. 29), the entrance to which is 

 guarded by a sphincter diaphragm. The endoderm lines the 

 whole canal system from the prosopyles to the osculum. In 

 several genera distributed among several orders the collars 

 which surround the base of the flagella are at their outer ends 

 fused to form a membrane, which was first described and 

 figured by Sollas (Fig. 30, 5). Bidder has shown that if a 

 Leucandra be placed in water with carmine suspended in it, 

 the water which comes from the oscula is always free from 

 carmine granules, thus showing the presence of a very efficient 

 filter, presumably the fused collars of the flagellate cells. It 



is still an open question whether 

 the space within this membrane, 

 between the body of the flagellate 

 cells, is empty, or occupied by a 

 transparent gelatinous substance. 



The mesoderm is a gelatinous 

 layer, with branched or stellate 

 cells scattered through it. Amoe- 

 boid cells (Fig. 31) wander through 

 its substance, and convey nutriment 

 from one part to another, and when 

 occasion arises assist in_ removing 

 irritant foreign matter from the 

 body of the sponge {phagocytes). 

 The branching mesoderm cells have 

 been traced into direct proto- 

 plasmic continuity with both 

 ectoderm and endoderm. The 

 reproductive cells are also meso- 

 dermal, and the fertilised ovum 

 develops in a space in the gelatinous mesoderm (9, Fig. 29), 

 which is lined by a layer of flat endothelial cells, also meso- 

 dermal in origin. 



Certain mesodermal cells in the neighbourhood of the 



Fig. 31. — Branchiug connective 

 tissue cells from the mesoderm 

 of Thenea muricata. After 

 Sollas. 



