THE HYDRA 



Phylum Ccelenterata Class Hydrozoa 



Exercise 1. — Occurrence and Collection. 



(a) Four species of Hydra are commonly found in the eastern and 

 central United States:^ Pelmatohydra oligactis, large, with a definite 

 stalk in the body region, with tentacles much longer than the body, 

 and brown in color; Hydra carnea, reddish brown or pinkish orange, 

 but smaller than the first-named species and having tentacles shorter 

 than the body, which is without a stalk; Hydra americana, no stalk, 

 color white or tan, tentacles shorter than the body and held erect; 

 and Chlorohydra viridissima, the small green hydra. The green ones 

 are not so favorable for study. Hydras live in clear ponds or slowly 

 moving streams and are most easily collected from small objects such 

 as plants and leaves. In the fall they are often found in great num- 

 bers attached to leaves that have settled to the bottom of a pond. 

 They can be collected from these with a pipette, or the leaves can be 

 brought to the laboratory and placed in aquaria where the hydras 

 can be seen attached to the vegetation or to the walls of the jars. 

 Examine such cultures showing the animals under approximately 

 natural conditions. 



Exercise 2. — -General Structure. 



(b) Study a hydra in a watch glass with sufficient water to allow 

 the animal to expand fully. Use a handlens or the low-power objec- 

 tive of the comi)ound microscope. The animal is attached by its base 

 or foot. The free end terminates in the hypostome, at the end of which 

 is the mouth which is closed except when the animal is feeding. 

 Tentacles surround the hypostome. How many tentacles are there? 

 Where do they arise? Note the knoblike clusters of nematocysts, or 

 stinging capsules, on the tentacles. What is their function? Observe 

 the movements and changes in shape of the animal; the handlens 



1 Pelmatohydra oligactis, Hydra americana, and Hydra carnea were formerly 

 confused, and all wore cla-ssified as H. vulgaris. The papers by Lihhip H. Hyman 

 in the Transactions of the American Microscopical Society, vols. 48, 49, and 50 

 (1929-1931), contain information concerning the identification of the hydras of 

 North America. 



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