CCELENTERATES 



197 



tion from the cnidoblast. The '' explosion " of the cell results in 

 the ejection of the dart and injection of the poisonous acid into 

 the victim. The animal hit by a number of these darts is usually 

 so paralyzed that further resistance is impossible. 



Food Taking. — The tentacles then reach out like arms, grasp 

 the food, and bend over with it toward the mouth. Digestion 

 takes place by means of a 

 fluid given out from some 

 of the cells near the mouth. 

 After the food has been 

 partially digested, many of 

 the cells lining the cavity 

 put out pseudopodia, which 

 grasp and ingest the food 

 particles. The tentacles are 

 hollow, and the body cavity 

 extends into them. From 

 this cavit}^ food may be 

 taken up by cells in all parts 

 of the body near where it is 

 to be used. The outer layer 

 of this little animal does 

 not digest the food, but 

 receives some of it already 

 digested from the inner t -^ ^- i .• r u ^ ^ u ^ i. ** u ^ 



^ Longitudinal section of a hydra; b, bud; 6a, attached 



layer. This food passes ^^d; m, mouth; ov, ovary; sp, spermary holding 

 r „ . 1, • 1 , sperm cells. 



irom cell to cell, as m plants, 



by osmosis. The oxygen necessary to oxidize the food is passed 

 through the body wall, seemingly at any point, for there are no 

 organs for respiration (breathing). 



Division of Labor. — We have here then a step toward a more 

 complex animal, for certain parts of the body here have certain 

 work to perform. The outside for sensation, offense and defense, 

 and for movement and coordination of parts, and the layer form- 

 ing the interior of the bag for taking in food, digesting it, and 

 distributing it to other parts of the body. 



Reproduction. — The hydra reproduces itself either by budding 



