l66 AQUATIC MICROSCOPY FOR BEGINNERS. 



be an inch long and easily visible to the naked eye; 

 when contracted it resembles- a minute globule of 

 green or of brown jelly, with the shortened arms at 

 the summit like small rounded knobs or projections. 

 It is exceedingly active so far as the arms are con- 

 cerned, for the body is always adherent to some sub- 

 merged object. The arms or tentacles are usually 

 stretched' out to their fullest extent, then often ex- 

 ceeding the body in length, and waving and twisting 

 about in search of prey. The figure (Fig. 130) shows 

 several Hydras nearly the natural size adherent to 

 Lemna rootlets. 



Fig. 130. — Hydras adherent to Lemna rootlets. 



The body is like a narrow cylindrical bag, the hollow 

 part of the little sack being the stomach, and commun- 

 icating directly with the external water, in which the 

 Hydra lives, by means of the mouth, around which are 



