248 



THE HYDRA 



shorter tentacles, and are likely to be found in greater numbers. 

 The brown hydras are sluggish, but their larger size renders 

 them more favorable for the study of the general organization. 

 In nature, hydras occur in much the same manner as in labora- 

 tory aquaria. The individual is attached at the basal region, 

 and extends into the water, tending to stand at right angles to its 

 surface of attachment with tentacles outspread. As observed in 

 an aquarium the green hydra extends its body and frequently 

 changes its position, while the brown hydra is more likely to remain 



with tentacles irregularly ex- 

 tended and almost motion- 

 less for much longer periods. 

 These different degrees of ac- 

 tivity are correlated with dif- 

 ferences in the feeding habits 

 of the two species. As may 

 be observed in the laboratory, 

 hydras devour small organ- 

 isms, such as water-fleas and 

 minute insect larvse, some- 

 times overpowering and ingest- 

 ing objects many times larger 

 than themselves. If given op- 

 portunity, hydras will feed 

 greedily upon bits of meat 

 and similar materials (Fig. 

 119). Although there are no 



. , , special organs of locomotion, 

 Vui 119.— Hydra with portion of body \ . .. 



' . ., , ;„ nttP,^ntin,T to the animal shifts its position 



turned inside out in attempting to ^ . 



ingest a piece of beef. from place to place by various 



simple movements of the body 

 (Fig. 120). Green hydras react positively to sunlight and thus 

 tend to collect in regions of optimum illumination. Before under- 

 taking a further description of habits and general behavior, it will 

 be necessary to describe the structures involved. 



General Structure and Body Plan. — The hydra, as shown by 

 Fig. 121, consists of an elongated body normally attached at one 

 end, the base, or " foot," and surmounted at the other by a circle 

 of tentacles enclosing a conical region, the kypostome, at the apex 

 of which is the mouth. The tentacles vary in number in different 



