260 THE HYDRA 



the cytoplasm of such cells contains a frothy or granular material 

 which can sometimes be seen in process of discharge, it is evident 

 that they are gland cells and are presumably responsible for the 

 digestive enzymes that act upon the food within the enteron. Just 

 inside the mouth there is a circlet of cells producing another 

 type of secretion which probably serves as a lubricant in the 

 swallowing of food, or perhaps as a paralyzing agent in addi- 

 tion to the nematocysts. Aside from the endodermal nerve cells 

 and sensory cells, described as part of the nervous system, no 

 other types of cells are known to occur in the endoderm. 



Hydra is, therefore, composed of two cell layers, the ectoderm 

 and the endoderm. This is a very simple type of organization and 

 one found only in the Coelenterata. In contrast with this, we shall 

 see that all the higher groups of animals have another cell layer, or 

 rather cell mass, the 7nesoderm, between the ectoderm and endo- 

 derm and composing the greater bulk of the body. This body 

 plan of hydra and the coelenterates is called diplohlastic in contrast 

 with the triploblastic plan which is found in higher forms. 



Metabolic Activities. — The capture and ingestion of food, 

 to which incidental references have been made, is discussed in the 

 section on behavior. After entering the mouth, the food is shifted 

 by peristaltic conti-actions of the body to a position in the upper 

 half of the enteron, where digestion occurs. As the food mass 

 is never found at lower levels, there is apparently a physiological 

 division of the enteron into a " stomach " portion and an " intes- 

 tinal " or absorptive one, although no structural difference is 

 recognizable, except the abundance of gland cells in the endoderm 

 of the upper body. If the objects ingested as food by a hydra are 

 examined during the earlier stages of digestion, it is found that 

 they are undergoing a disintegration similar to that which takes 

 place in the stomach of any carnivorous animal. The softer parts 

 of a water-flea, for example, become liquefied and divided into 

 smaller particles, while fragments of the indigestible skeleton 

 remain as would bits of the skeleton of an insect or a crayfish 

 within the stomach of a frog. Eventually, these indigestible 

 portions are egested, slowly or by a violent contraction, through 

 the mouth. It is thus evident that a digestive process takes 

 place within the enteron. This is doubtless caused by enzymes, 

 since these are present in all cases where digestive juices 

 can be collected in sufficient quantity for experimentation. 



