62 MANUAL OF ANIMAL BIOLOGY 



large cavities appear in the cytoplasm. The glandular endo- 

 derm cells, which secrete the enzymes into the enteric cavity, 

 exhibit no noteworthy structural modifications from those found 

 in a typical animal cell. Interstitial cells are also found at the 

 bases of the endoderm cells, next to the mesogloea. 



B. Life Processes 

 1. Nutrition 



Hydra is chiefly a carnivorous animal, feeding upon a great 

 variety of small, aquatic animals. These are first paralyzed by 

 the nematocysts and then conveyed by the flexible, muscular 

 tentacles to the central mouth opening. The ectoderm cells of the 

 tentacles are able to secrete a sticky substance which aids in hold- 

 ing the captured prey. (B. f. 5.) 



The food having been ingested, that is, passed through the mouth 

 into the enteric cavity, the process of digestion begins This may 

 be either intracellular or intercellular, as noted above. It is prob- 

 able that most of the food of Hydra is digested by the intercellular 

 method. Thus to the endoderm cells is entrusted the nutrition of 

 the entire Hydra, a condition which is typical of all the higher 

 animal types. In the larger and more highly developed forms of 

 animals a specialized transport tissue, the blood, is necessary in 

 order to carry the food absorbed from the alimentary canal to all 

 the other cells of the animal body. In Hydra, however, this is not 

 necessary, and the soluble food is passed along from cell to cell by 

 diffusion through the cell walls. 



A certain proportion of the material ingested by a Hydra con- 

 sists of inorganic material which is not capable of being digested. 

 These indigestible substances are egested from the enteric cavity to 

 the exterior through the mouth opening. This is the only opening 

 into the digestive cavity of Hydra, and it serves as a common open- 

 ing for the ingestion of food and for the egestion of the refuse. 



Symbiosis. Among the several species of Hydra, the green 

 colored species, Chlorohydra viridissima, is of particular interest 

 from the standpoint of nutrition because of the fact that many of 

 the endoderm cells of this species actually contain a great number 

 of tiny, green plant cells belonging to the unicellular Alga, Chlorella 

 vulgaris, which is closely related to Pleurococcus (p. 11). The 

 cells of this plant give this species of Hydra its green color. 



