MORPHOLOGY OF ANIMALS 



plasm itself. After the substances rendered soluble by- 

 digestion have been removed from the food, the indigesti- 

 ble remnants are cast out of the body in solid form 

 {egestion). 



Intracellular Digestion. Among the Protozoa digestion 

 occurs within the body of a single cell, that is, it is intra- 

 cellular. The same is true of sponges and some Cnidaria, 

 in which the food, consisting of microscopic particles, is 

 ingested and digested by certain epithelial cells, lining the 

 endodermal cavities. In all animals above the sponges 

 intracellular digestion is limited to the endoderm cells 

 and to certain free cells, such as white blood-corpuscles 

 (leucocytes^ and it is of decreasing importance as one 

 ascends the scale. 



Extracellular Digestion. In all animals except the 

 lowest, digestion occurs principally in a digestive cavity 

 surrounded by cells which pour their secretions into the 

 cavity. By the action of these secretions certain insoluble 

 food substances are transformed into soluble ones. This 

 digestive cavity is in all cases derived from the enteron or 

 primitive digestive cavity of the gastrula, and in the sim- 

 plest cases it is little more than a sac whose walls may be 

 folded into ridges or septa, thus enlarging the digestive 

 surface (Anthozoa), or they may be extended to form 

 tubular canals, the gastro-vascular system, by means of 

 which the digested food is also distributed to all parts of 

 the animal (Scyphozoa, Ctenophora, Turbellaria, Fig. 

 11, A). 



Openings into Digestive Cavity. In all Cnidaria except 

 the lowest class, and in all animals above the Cnidaria, 



C 35 3 



