182 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



smaller and smaller fragments. Digestion appears to be aided by 

 the churning movements caused by expansion and contraction of 

 the body wall. Ultimately some of the food is reduced to a soluble 

 state, and absorbed into the endodermal cells. Meanwhile some of 

 the large vacuolated cells put out pseudopodia and engulf some of 

 the undigested food particles, finishing the digestive process inside 

 their own cell-bodies. Thus Hydra has two types of digestion, one 

 intracellular, like that found in all unicellular animals and, there- 

 fore, more primitive ; the other, extracellular, that is, taking place in 

 the digestive cavity. Most of the food of the Hydra is digested in 

 the latter way, the cells lining the cavity absorbing the digested food 

 before passing it along to the cells of the ectoderm. According to 

 Hegner, part of the absorbed food is in the form of oil globules which 

 are passed over to the cells of the ectoderm and stored there for future 

 use. Unusable or undigested material is thrown out of the digestive 

 cavity by a sudden contraction, there being no other way of eliminat- 

 ing such wastes except through the surface of the body, as in lower 

 forms. Hydra like other animals uses oxygen to release energy. 

 Respiration probably takes place through the surface of the entire 

 body, the cells receiving oxygen and giving off carbon dioxide by 

 diffusion through the cell membranes. 



Reactions to Stimuli 



Hydra show very definite reactions to certain stimuli, most of 

 which have to do with obtaining food. Hungry Hydra are much 

 more active than well-fed ones, and respond to various chemical 

 stimuli besides reacting to mechanical stimuli, to heat, to light, and 

 to electricity, all of which indicates the possession of some sort of 

 simple nervous system, since the movements made are more or less 

 co-ordinated. If touched lightly on a tentacle with a needle, only 

 the tentacle contracts, but with increased stimulation, the other 

 tentacles contract, until finally, the whole animal draws down into 

 a little ball. Its physiological condition, according to Jennings,^ 

 determines whether it " shall creep upward to the surface and toward 

 the light, or sink to the bottom ; how it shall react to chemicals and 

 to solid objects ; whether it shall remain quiet in a certain position, 

 or reverse this position and undertake a laborious tour of exploration." 



The nervous system of Hydra forms a nerve net. It consists of a 

 concentration of primitive nerve cells about the base of the hypostome 



1 Jennings, Behavior of the Lower Organisms. Columbia Univ. Press, 1915, p. 231. 



