GRANTIA 55 



Altogether there are some thirteen different types of cells present 

 in the three layers of this sponge, all of which perform essential 

 functions in the economy of the organism. Summarizing the 

 duties of the various types of cells, it may be said that those of the 

 dermal region are largely concerned with the protection and the 

 support of the body tissues through the formation of the covering 

 and skeletal elements. The middle region contains the amoeboid 

 cells which are concerned with the transportation of nutritive ma- 

 terials and reproduction, while the gastral region contains cells of 

 only one type, the collared, flagellated choanocytes, which are 

 chiefly nutritive in function. 



B. Life Processes 



From the preceding description of the chief structural features 

 of Grantia, it is evident that, in general, the cells are structurally 

 modified and variously adapted for the particular needs of the 

 animal. In agreement with the structural modifications, the cells 

 are also functionally specialized in certain instances, as, for ex- 

 ample, in the case of those forming the supporting elements, to 

 such an extent that they are incapable of performing all the life 

 processes necessary for the maintenance of the organism as a whole 

 as are the balanced, independent cells of a multicellular protozoon 

 like Volvox. In Grantia, as elsewhere, cell specialization is in- 

 evitably associated with a division of labor between the dif- 

 ferentiated cellular groups, each of which is concerned with a 

 certain function or functions necessary for the existence of the 

 complete organism. Such a condition makes it necessary that 

 each cell group share in the benefits derived from the activities 

 of all the other specialized cell groups. 



1. Nutrition 



Grantia as a holozoic organism is dependent for its food supply 

 upon organic substances in a solid form. These it finds in a very 

 finely divided condition in the water current which, as has been 

 noted, is continually flowing through the gastral cavity of the 

 organism. It will be recalled that this flow of water is due to the 

 coordinated actions of the flagella of the choanocytes which line 

 the radial canals of this organism. 



But these choanocytes have not merely a mechanical function in 

 nutrition, they are also the agents for the ingestion and digestion 



