56 MANUAL OF ANIMAL BIOLOGY 



of the food particles. These functions they perform in essentially 

 the same manner as do protozoan cells ; that is, the food is actually 

 engulfed, or ingested, and the process of digestion takes place 

 within the body of each individual cell. This constitutes intra- 

 cellular digestion and is to be differentiated from intercellu- 

 lar digestion present in the higher types of Metazoa in which 

 certain of the cells lining the digestive tract secrete the digestive 

 fluids, or enzymes, into a common cavity where the food is di- 

 gested, and from which it is later absorbed for distribution among 

 all the cells of the organism (pp. 85-87). 



Thus we have the choanocytes of Grantia securing the nutritive 

 materials for the entire organism. It may be asked at this point 

 as to how the other cells of the organism secure the necessary supply 

 of the food captured and digested by the choanocytes. It is known 

 that the excess of nutrient materials is given up by the choanocytes 

 for general distribution. It appears to be a function of the amoe- 

 boid wandering cells of the middle layer to receive and transport 

 these substances to the various other types of cells. 



2. Respiration and Excretion 



There are no specialized cells in Grantia for the performance 

 of the respiratory and excretory functions of the entire organism. 

 Accordingly each cell is responsible for the performance of these 

 necessary cellular functions. However, the more favorable posi- 

 tion of the cells of dermal and gastral regions, with respect to 

 the surrounding oxygen-containing water, as compared with the 

 enclosed cells of the middle region, indicates that the latter cells 

 pick up their oxygen from, and pass off their wastes through, the 

 dermal and gastral cells. Here again it is apparent that the cells 

 of Grantia are close to the protozoan level with respect to these 

 two functions. 



3. Reproduction 



Grantia may reproduce asexually by budding, or sexually by 

 the production of highly organized germ cells which have their 

 origin in the middle region of the body. In asexual reproduction, 

 rapidly dividing cells, apparently originating from all three layers 

 of the body, gradually form a small outgrowth, or bud, near the 

 base of the parent animal. The bud gradually enlarges and differ- 

 entiates to form the fully developed individual. Frequently it 



