BEHAVIOR OF CCELENTERATA 21 1 



the lower side become more turgid, increasing in volume and thus bend- 

 ing the stem directly upward. Either the entire animal or a piece of 

 the stem, without head or foot, reacts in this manner. Thus the reaction 

 is in this animal comparable to the reaction to gravity in a plant. 



But in many species of fixed ccelenterates gravity clearly has "little 

 or nothing to do with the usual position. Metridium, Aiptasia, Stoi- 

 chactis helianthus, Condylactis passiflora, and many others are found 

 occupying all sorts of positions with reference to gravity, and the same 

 is true of Hydra and various hydroids. 



In some medusae the movement is partly guided by gravity. Go- 

 nionemus, as we have seen, swims in its "fishing" movements upward 

 to the surface. Yerkes (1903) found that this occurs in the same way 

 when the light comes from below, so that the guiding factor is apparently 

 gravity. This reaction to gravity is of course not constant; it occurs 

 only at intervals and under certain circumstances. 



Careful examination will probably show that gravity plays a part in 

 certain episodes of the behavior of most of these animals, even though 

 it may not affect their usual position or direction of motion. Thus, 

 gravity plays a part in the "rejecting reaction" of the actinian Stoi- 

 chactis, described in Section 5 of the present chapter. The situa- 

 tion " waste-matter-on-the-disk-not- removed- by-the-first-reaction " is 

 responded to by taking such a position with reference to gravity as re- 

 sults in removing the waste ; then the reaction to gravity ceases. Simi- 

 lar transitory reactions to gravity, seeming to serve definite ends, are 

 found in many other animals. Thus, in the hermit crab, according to 

 Bohn (1903), we have such a case. While investigating a shell which 

 it may adopt as a home if fitting, this animal takes a certain position 

 with reference to gravity ; namely, with body on the steepest slope of 

 the shell, and head downward. It then turns the shell over (the posi- 

 tion mentioned being the most favorable one for this action), and ceases 

 to react with reference to gravity. Other cases of the same sort will be 

 described for the flatworm Convoluta (Chapter XII). Gravity has, of 

 course, many diverse effects on the substance of organisms, and in al- 

 most no case has its precise action in directing movements been deter- 

 ' mined. When an animal is inverted, this may cause a redistribution 

 of the constituents of the body or of the separate cells. Such a redis- 

 tribution would probably interfere with the usual physiological pro- 

 cesses, and might therefore act as a stimulus to a change of position. 

 Again, in freely moving organisms, gravity causes differences in the 

 ease of movement in different directions, and such differences may 

 well determine the direction of motion. Again, a change in the usual 

 position with reference to gravity may induce unusual strains in various 



