HYDRA FUSCA 141 



attached to the lily leaf, be removed from the pond, placed in 

 a dish of water and left undisturbed for a time, it will slowly ex- 

 pand and assume the form represented in Figure 69 A. It shows 

 then a slender body about a quarter of an inch or less in length, 

 attached at one end to some other solid object. At the other 

 end it bears a crown of tentacles, which in the brown Hydra 

 are from five to ten in number, and in the green Hydra are from 

 five to twelve. These tentacles are very delicate, hairlike bodies, 

 which may be expanded to considerable length, as at A, but when 

 contracted, shrink into minute knobs hardly big enough to be 

 seen. Indeed, the whole body of the Hydra is extremely con- 

 tractile, and though when undisturbed it may be a half an inch 

 or more in length, on being disturbed it will contract into a 

 small body no larger than a pinhead; see Fig. 69 B. Hydra 

 seems at first to be a stationary animal, although it can move 

 its tentacles slowly to and fro in the water. A careful examina- 

 tion, however, shows that it has some power of motion; the ani- 

 mal, creeping by means of its base, can move slowly over the 

 object upon which it is fastened. Occasionally also it moves 

 by turning end over end. It first attaches its tentacles to the 

 object to which its base is attached. Then the base lets go its 

 hold and is moved over and fastened again in another spot. 

 The tentacles let go their hold and the animal straightens up. 

 The movement is not unlike that of a boy turning a handspring. 



Structure. In the midst of the crown of tentacles is a little 

 conical projection, on the top of which is a mouth. This is 

 star-shaped rather 'than circular, and opens into a cavity which 

 fills the whole of the body of the Hydra and even extends into 

 its tentacles. This cavity is the digestive cavity and is called 

 the gastrovascular cavity; see Fig. C. 



Hydra is a true multicellular animal, made up of many thou- 

 sands of cells which are not alike but show a considerable differ- 

 entiation and have a division of labor among them. All of these 

 cells, however, are arranged into two layers, one on the out- 

 side called the ectoderm (Gr. ectos = outside + derma = skin), ec 



