Branch Ccelenterata 2i 



it is likely to pass the food to the base of the tentacle, 

 where, in the middle of a round projection, the mouth, 

 opening into the body cavity, may be seen. 



V. — Make another careful study of the Hydra, endeavoring 



to see the hollow nature of its body and tentacles. This 

 may sometimes be done by strong light when the 

 animal is extended. Is the body cavity continued into 

 the tentacles .' Encircled by the bases of the ten- 

 tacles, see a cone-shaped extension of the body, the 

 hypostome. How is the mouth situated with reference 

 to the hypostome .' Unless you are fortunate in retain- 

 ing the specimen in the right position, it will require 

 careful manipulation with needles or other instruments 

 in order to see clearly the mouth opening. 



VI. — See that the walls of the Hydra's body and tentacles 

 are made up of two layers of cells : the inner layer is 

 called the endoderm, and the outer layer, the ectoderm. 

 The colored appearance of the green Hydra is due 

 to the pigmented chlorophyl. Focus carefully with 

 strong light along the edge of the body or tentacles, to 

 determine whether this chlorophyl is contained within 

 the ectoderm or the endoderm. Make a drawing, to 

 show the double or diploblastic nature of the body wall. 



VII. — Notice that the mouth furnishes the only entrance 

 to the body cavity, which .serves as a stomach. Food 

 is pushed in by the tentacles through the mouth, and 

 through it the indigestible matter is ejected by the 

 contraction of the body. Note the fact that the food 

 comes in contact with the endodermal cells only. 

 These cells alone perform the function of digestion, and 

 thereby exhibit a true division of labor among the cells 

 of the organism. The ectodermal cells obtain their 

 food only after it has been digested and passed through 



