METAZOA 



39 



and into the central cavity, thence it leaves through the 

 oscula. 



Embryological research shows that vs^e 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 lining 

 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 coatiUg 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 



3- 



Fig. 30. — Stelospongus 

 fldbeUi/ormis. 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. 



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 



