USE OF THE MICROSCOPE IN ZOOLOGY. 51 



consisting of spines and filaments, supposed by some 

 to have the power of stinging. There are traces of 

 muscular fibres in the tentacles, but whether sufficient 

 to account for their extraordinary extensibility is 

 doubtful. Some have supposed their elongation to be 

 caused by the water, which, finding its way into the 

 hydra's body through the mouth, may pass through 

 extremely narrow channels into the tentacles. The 

 tentacles of Hydra fusca are the most wonderfully 

 extensible of all; growing gradually finer than the 

 lightest gossamer, they become invisible except to 

 the eye of the microscopist. 



The hydrae are very voracious, and readily kept in 

 confinement for some time. They feed on small 

 Entomostraca ', and on minute larvae of gnats and naid 

 worms. Their stomach is a simple cavity. Some 

 authors speak of a short narrow duct leading from the 

 stomach to the centre ot the disc, whence they say 

 through a tiny aperture excrementitious particles may 

 be seen to pass. Of this intestinal canal I have never 

 discovered any sign in the species I have examined. 

 Food is quickly assimilated by the hydra, and the 

 indigestible portion expelled through the mouth, as 

 in the Actinice. The movements of the hydra are 

 very slow, but performed in the same manner as those 

 of the leech, their position being also varied by a 

 gliding motion of the disc. Sometimes this disc, 

 protruded above the water, acts as a float, and the 

 animal is borne along on the current. Hydrae may be 

 found in spring, summer, or autumn, and in the latter 

 season they give birth to eggs and die. I have found 

 H. viridis in very mild winters. Their mode of in- 

 crease is twofold gemmation and the ordinary mode 

 of reproduction. The first takes place throughout 

 the summer, the latter only at the end of autumn. 

 When increasing by gemmation, a small swelling first 



D 2 



