IV _ HYDRA 291 



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

 section shows that the body is hollow, containing a spacious 

 cavity, the enteron (Fig. 74, a, ent. cav), which communicates 

 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. 73). 



An examination of the living animal shows, in the first 

 place, that its form is continually changing. At one time (Fig. 

 73, 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 expansion. 

 Hydra is able to move slowly from place to place. This it 

 usually does after the manner of a looping caterpillar (Fig. 

 73, 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 like a cuttle-fish (d) 



u 3 



