32 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



The green species is common on species of Vaucheria 

 or greenfelt, which grow in rapidly flowing creeks. 

 Mats of the plant may be put into white earthenware 

 dishes. After a few minutes the hydras will expand 

 and be easily seen against the background formed 

 by the dish. 



Aquarium Study. With the naked eye or with a 

 lens study the hydra in situ, noting its color, shape, 

 size, the body and the tentacles, the number and 

 extensibility of the latter; touch the body or the ten- 

 tacles with a bristle and note the sensitiveness of the 

 animal. Look for individuals bearing buds ; the num- 

 ber and position of the latter; the radial symmetry 

 of the body. Note how well the shape and color 

 of the animal adapt it to its surroundings (Fig. 17). 

 ' Microscope Study. Transfer a fragment of the 

 plant bearing a hydra to a watch glass (or, if the 

 animal is fastened to the side of the jar, detach it 

 with a pipette), and examine under a lens or under 

 a low power. Note again the movements of the 

 body and tentacles. . Put a minute fragment of fresh 

 meat within reach of the tentacles and endeavor to 

 see the hydra catch and swallow it. Study the move- 

 ments of the mouth. 



Mount a specimen under a cover glass arM study 

 the structure of the body, its walls consisting of two 

 layers of cells ; the central cavity with one exterior 

 opening ; the color of the inner cell layer and its 

 cause ; the structure of the tentacles, the groups of 

 nettle cells; cause the discharge of the latter by run- 

 ning a drop of weak acetic acid under the cover glass. 

 On individuals bearing buds study the structure and 

 actions of the latter ; their mode of attachment to 

 the parent ; the small " colony " formed by this mode 

 of reproduction (Fig. 18). 



