9 6 



COLLEGE ZOOLOGY 



Spicules are built up within cells called scleroblasts, which form 

 part of the inner stratum of the dermal layer. 



Physiology. — Grantia lives upon the minute organisms and 

 small particles of organic matter that are drawn into the incur- 

 rent canals by the current of water produced by the beating of 



the collar-cell flagella. 

 The majority of the 

 food particles are en- 

 gulfed by the collar 

 cells. Digestion, as in 

 the Protozoa, is intra- 

 cellular, food vacuoles 

 being formed. The dis- 

 tribution of the nutri- 

 ment is accomplished 

 by the passage of 

 digested food from cell 

 to cell, aided by the 

 ameboid wandering cells 

 of the middle layer. 



Excretory matter is 

 discharged through the 

 general body surface, 

 assisted probably by 

 the ameboid wandering 

 cells, and possibly by 

 the collar cells, also. 

 Respiration likewise 

 takes place, in the ab- 

 sence of special organs, through the cells of the body-wall. 



Reproduction. — Reproduction in Grantia takes place by 

 both sexual and asexual methods. In the latter case, a bud 

 arises near the point of attachment, finally becomes free, and 

 takes up a separate existence. 



The sexual reproductive cells lie in the jelly-like layer of the 



Fig. 56. — A simple sponge, Sycon. The 

 right-hand member of the colony is shown in 

 longitudinal section. ip, incurrent pores; 

 o, osculum. (From Parker and Haswell.) 



