CHAPTER XVIII. 

 THE HYDRA. 



THE fresh-water polyp (Hydra viridis) may be found in 

 summer attached to duckweed in ponds. If some of the duck- 

 weed be placed in a glass vessel near a window, the little green 

 polyps will congregate to that side of the vessel which is turned 

 towards the light. They are minute gelatinous bodies, half an 

 inch or less in length. Each is attached to the glass or to the 

 duckweed by a flat disc, known as the foot. When the hydra 

 is fully extended the foot is more expanded than the body, but 

 when the organism is in a state of contraction the foot is 

 narrower than the rounded and swollen body. At the end of 

 the body opposite to the foot is the hypostome (Fig. 99, h. s.), 

 bearing at its summit the mouth. Encircling the hypostome 

 are the tentacles (^.), varying in number according to the age 

 of the individual, but seldom exceeding eight. By alternately 

 applying the foot and the tentacles, the hydra can creep with a 

 looping motion. The body surface generally, and the tentacles 

 especially, are irritable. If one of the tentacles be touched, it 

 is rapidly withdrawn, the irritation spreads to the others, and 

 they too are drawn towards the body, which itself contracts, so 

 that the whole organism, to the naked eye, looks like a rounded 

 speck of green jelly. In Fig. 99, C., the hydra is represented in its 

 fully elongated condition ; in A. it is moderately contracted ; in 

 B. in the condition of utmost contraction. After irritation the 

 contraction relaxes, and the hydra resumes its fully expanded 

 state. Should any minute animalcule or small water-flea come 

 in contact with one of the tentacles, it seems to stick, and 



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