146 BIOLOGY 



from their free surface into the cavity of the body, and is thus 

 poured upon the food which is taken into the mouth and lies 

 free in the gastrovascular cavity (intercellular digestion). 



Hydra has thus, in addition to a method of digestion which 

 resembles that of the Amoeba, the power of producing a digestive 

 secretion, which is poured upon the food in the general cavity, 

 only the nutritious portions of the food being absorbed after 

 digestion. This method of digestion, which is peculiar to the 

 higher animals, is, in the Hydra, combined with the simple 

 method of digestion characteristic of the PROTOZOA; and in 

 this respect the Hydra represents a transition stage between the 

 unicellular animals and the higher, multicellular forms. The 

 function of the hairlike flagella on the endodermal cells appar- 

 ently is to keep in circulation the liquids present in the body and 

 thus to aid in bringing the digestive juices in contact with the 

 food which lies in the cavity. This is the only trace of a circu- 

 latory system that the Hydra possesses. 



Nervous System. According to recent investigation, it seems 

 that Hydra possesses a very simple nervous system, so delicate, 

 however, that it requires special methods of study; and very 

 little is known about it. There is a series of nerve cells near 

 the mouth and another near the base of the animal, and these 

 are connected with excessively delicate fibers passing over the 

 body. There are sensory cells on the surface layer that are 

 probably connected with the nerve cells, and some of the nerve 

 cells apparently send nerve fibers to the contractile fibers of the 

 epithelio-muscle cells. This system is, however, very simple 

 and rudimentary, and is of interest chiefly as the simplest type 

 of nervous system found among animals. 



Growth and Budding. The food of Hydra consists mainly 

 of minute water animals which are captured by means of its 

 tentacles. The tentacles are protruded into water, and small 

 animals, coming in contact with them, are paralyzed by the 

 discharge of the nematocysts. The tentacles then transfer the 

 food to the mouth. It is pushed into the gastrovascular cavity 



