296 HYDRA CHAP. 



(t) arranged in a circlet or whorl. A longitudinal section 

 shows that the body is hollow, containing a spacious 

 cavity, the enteron (Fig. 76, A, ent. cav), which commu- 

 nicates with the surrounding water by the mouth. The 

 tentacles are also hollow, their cavities communicating 

 with the enteron. 



Thus it will be seen that the Hydra is not bilaterally 

 symmetrical, like the frog i.e., equally divisible into two 

 lateral halves by a median vertical plane passing through 

 the axis of the body but is radially symmetrical, i.e., the 

 body is divisible into similar parts radiating from a 

 common central axis. 



There are three kinds of Hydra commonly found : one, 

 H . vulgaris, is colourless or nearly so ; another, H. fusca, 

 is of a pinkish-yellow or brown colour ; the third, 

 H. viridis, is bright green. In the two latter it is 

 quite evident, even under a low power, that the colour 

 is in the inner parts of the body-wall, the outside of 

 which is formed by a transparent, colourless layer 



(Fig. 75). 



An examination of the living animal shows, in the 

 first place, that its form is continually changing. At 

 one time (Fig. 75, A, left-hand figure) it extends itself 

 until its length is fully fifteen times its diameter and the 

 tentacles appear like long delicate filaments : at another 

 time (right-hand figure) it contracts itself into an almost 

 globular mass, the tentacles then appearing like little 

 blunt knobs. 



Besides these movements of contraction and expan- 

 sion, Hydra is able to move slowly from place to place. 

 This it usually does after the manner of a looping 

 caterpillar (C) : the body is bent round until the distal 

 end touches the surface : then the base is detached and 

 moved nearer the distal end, which is again moved 

 forward, and so on. It has also been observed to crawl 



