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. 51) 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- 
Fic. 31.—Branching connective plasmic continuity with both 
cuore’ See gee ectoderm and endoderm. The 
Sollas. 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 
