IV] 



THE SECOND GRADE 



91 



however it is very much in evidence, beating several 

 times a second, and so producing the current, from 

 which food is taken up and digested by the collar- 

 cells. The bases of these cells rest upon a thin 

 layer of jelly dead stuff secreted by the living cells, 



Fig. 6. Clathrina coriacea, Mont. Two sections of the body-wall. 

 E, not quite fully expanded ; the collar-cells line the cavity of 

 the sponge, and show collar and flagellum. F, very much con- 

 tracted. The collar-cells have withdrawn collar and flagellum, 

 and are lying in irregular masses behind the layer of immigrated 

 pore-cells, am.c, amoebocytes ; c, collars of choanocytes (ch.c) ; 

 d.ep, dermal epithelium ; fl, flagella; p.c, pore-cells; sp.c, spicule- 

 cells. (Highly magnified.) (From Minchin.) 



and serving, like the somewhat similar gelatinous 

 tissue we shall see in Volvox, for the common support 

 of the separate cells. On the outside of the jelly is 

 the dermal layer of flat polygonal cells, fitting 

 together like a mosaic of tiles. The pores through 

 which the current enters are perforations in the 



