CHAPTER XXIX. THE FRESH- WATER POLYPE. 



(Hydra vidgaris, H. viridis, H. fusca t ) 



1. Occurrence and Mode of Life. Hydra is a common 

 inhabitant of fresh-water ponds in which duck-weed (or 

 other water-plants) and small crustaceans are abundant. 

 If a glass jar be filled by dipping it into such a pond and 

 left in a light place for an hour or so, Hydrse are likely to 

 be found sticking to the sides of the 

 jar in an excellent position for ex- 

 amination with a hand-lens. If 

 fresh water is added every now and 

 then to make up for loss by eva- 

 poration, the Hydrse can be kept 

 alive for an indefinite time. 



Each Hydra is a cylindrical body 

 usually attached to water- weed (or 

 the sides of the vessel) by one end, 

 and having at the other end a variable 

 number of long, fine tentacles (see 

 fig. 193). If roughly touched, con- 

 traction takes place, resembling that 

 of a muscle the body becomes shorter 

 and thicker almost globular and 



,-111-1 i_ j. i it, i Fig- 193. Two HYDR.E 



the tentacles very short and thick ATTA CHED TO WATER-WEED. 



(fig. 193, B). Very SOOn, however, it A with two buds, expanded, 

 rpaniTi<5 if<? normal :hnr>A fWntsirm and holding a water-flea. B, 



regains its noimai snape. occasion- contracted * (After p arker .) 

 ally a Hydra may be seen changing 



its place, moving very slowly, after the fashion of a 

 looping caterpillar. Its normal position, however, is the 

 fixed one, and it affords us the only example among our 

 types of a fixed animal. The fixed habit of life, which we 

 usually think of as one of the most distinctive features of 

 plants, is, however, by no means uncommon among aquatic 

 animals, and is very often accompanied by another plant- 

 like habit, viz. that of branching (fig. 196 and 14 below). 



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