METAZOA — 39 
and into the central cavity, thence it leaves through the 
oscula. 
Embryological research shows that we must regard the collar 
cells of the flagellate chambers, the cells lining the tubes which 
lead from them to the central cavity, and the cells lning 
the latter cavity, as endoderm. The mesoderm is a gelatinous 
tissue in which certain cells are found embedded; some of 
these form ova or break up into spermatozoa, whilst others give 
rise to the skeleton of calcareous spicules. 
In sections of Grantia the intercanal spaces (3, Fig. 29) 
may be seen lying between the flagellate chambers, but quite 
distinct from these. They are lined by flat epithelial cells, 
and they ultimately open by a more or less wide mouth on to 
the exterior on the one hand, and by a series of circular pores, 
the prosopyles, into the flagellate chambers. These intercanal 
spaces are formed by the pushing in of the outer coating of the 
sponge, and are lined by ectodermal cells similar to those 
covering the outside of the sponge. In some sponges they 
reach a great degree of complexity. 
The ectoderm of sponges is, as a rule, composed of flat cells 
j} ] 
f flabelliformis. Diagram 
of a portion of a flagel- 
late chamber, showing 
the various parts of the 
collared cells and their 
relation to  Sollas’s 
membrane, which con- 
nects together the mar- 
gins of the collars. 
After Dendy. 
( Fic. 30. — Stelospongus 
1. Body of flagellate cell. 
2, Its nucleus. 
3. Its collar fusing at its 
edges to form 5, 
4, The flagellum. 
5. Sollas’s membrane, 
in a single layer; rarely these cells become columnar, and bear 
flagella (Oscarella lobularis). The endoderm consists, in all 
but the Homocoela, of two kinds of cells: (i.) flat pavement 
