GENERAL CHARACTERS 221 



ation with a pocket lens shows that from the free extremity 

 a number of very delicate filaments, barely visible to the 

 naked eye, are given off. 



Under the low power of a compound microscope, a Hydra 

 (Fig. 51, B) is seen to have a cylindrical body attached by a 

 flattened base to a weed or other aquatic object, and bearing 

 at its opposite or distal end a conical structure, the hypostome 

 (Jiyp\ at the apex of which is a circular aperture, the mouth 

 (;;////.). At the junction of the hypostome with the body 

 proper are given off from six to eight long delicate ten- 

 tacles (/) arranged in a circlet or whorl. A longitudinal 

 section shows that the body is hollow, containing a spacious 

 cavity, the enteron (Fig. 52, A, ent. cav), which communicates 

 with the surrounding water by the mouth. The tentacles are 

 also hollow, their cavities communicating with the enteron. 



There are three kinds of Hydra commonly found : one, 

 H. vulgar is ) 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. 51, A, B). 



It is quite easy to keep a Hydra under observation on the 

 stage of the microscope for a considerable time by placing it 

 in a watch-glass or shallow " cell " with weeds, &c., and in 

 this way its habits can be very profitably studied. 



It will be noticed, in the first place, that its form is 

 continually changing. At one time (Fig. 51, 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. 



