SPONGES, FLAT WORMS, AND ROUND WORMS 147 



blasts, which produce spicules, are also considered constituents 

 of the dermal layer. 



The inner epithelium lines the cloacal cavity and the radial 

 canals. In the latter it consists of one layer of collared flagel- 

 lated cells (Fig. 71, end.), termed choanocytes. No collar cells 

 are present in the 

 epithelium -lining the 

 cloacal cavity. The 

 flagella of the collar 

 cells create the cur- 

 rent of water which 

 is continually flowing 

 through the body wall 

 into the cloacal cavity 

 and out of the oscu- 

 lum. 



The middle region of 

 the body wall is not e 

 so definite nor firm 

 in structure as are the 

 outer and inner epi- 



FIG. 72. Section of a portion of Grantia. I, open- 

 ings of incurrent canals ; 2, incurrent canal ; 

 3, prosopyle ; 4, radial canal ; 5, choano- 

 cytes ; 6, spicules ; 7, opening of radial canal. 

 (From Shipley and MacBride after Dendy.) 



FIG. 73. Develop- , . . . ., , 



ing spicules of thelia ' but ' nevertheless, it is considered 



a sponge. a distinct cellular layer. Ameboid wan- 



(From Dahl- dering cells, which ingest food or act as 



grenandKep- storage cells, are found here, as well as 



ner after A. ^ re p ro ductii}e cells which always arise 

 Maas.) . , ... . 



in the middle layer. 



The soft body wall of Grantia (Fig. 72) is supported and pro- 

 tected by a skeleton composed of a great number of spicules of 



