PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 



113 



and sponge in that it is carried on outside 

 the cell, that is, extracellular. However, in- 

 tracellular digestion also takes place in the 

 hydra; the pseudopodia thrust out by the 

 gastrodermal cells (Fig. 52) seize and en- 

 gulf particles of food; these particles are 

 then further digested in the cells. The di- 

 sested food is absorbed and stored by the 

 g^strodermis. 



One species of hydra, Chlorohydra viridis- 

 sima, is green in color because of the pres- 

 ence of a unicellular alga, Chlorella vulgaris, 

 in the gastrodermal cells. As in Paramecium 

 bursaria, the plant uses some of the waste 

 products of metabolism of the hydra, and 

 the hydra uses some of the oxygen resulting 

 from the process of photosynthesis in the 

 plant. This condition is one of mutualism. 



Egestion 



All indigestible material is egested from 

 the mouth. This is accomplished by a very 



Hanging fronn the 

 surface of water 



sudden squirt which throws the debris some 

 distance. 



Respiration and excretion 



Oxygen diffuses into the cells from the 

 water in which the hydra lives, and carbon 

 dioxide diffuses out of the cells. The met- 

 abolic waste products are excreted through 

 the general body surfaces. 



Behavior 



Spontaneous movements 



All the movements of the hydra are the 

 result of the contraction of the contractile 

 fibers and are produced by two kinds of 

 stimuli, internal, or spontaneous, and ex- 

 ternal. Spontaneous movements may be ob- 

 served when the animal is attached and un- 

 disturbed. At intervals of several minutes, 

 the body, or tentacles, or both contract sud- 

 denly and rapidly, and then slowly expand 



Locomotion by a series 

 of somersaults 



Figure 55. Sketches showing the hydra feeding and its methods of locomotion. Somersaulting 

 is the most rapid method of locomotion. 



