468 H. S. 'Jennings. 



in the sand — in its natural habitat — is it at rest. If this condition 

 is fulfilled, the requirement of the usual position in line with 

 gravity may be neglected. Loeb found that when the animal is 

 placed in a test tube, so that its body is in contact with the sides, 

 it remains here indefinitely, even though the tube is placed in a 

 horizontal position (Loeb, 1891, p. 54). The head is bent upward, 

 but the body remains transverse to the direction of gravity. 



Examples of the fact that a certain orientation with reference to 

 gravity is not a rigid requirement even in animals that usually or 

 at times react to this agent, are common among sea anemones and 

 other lower organisms. Thus, Torrey ('04) shows that Sagartia, 

 though it usually maintains an upright position, may ofttimes take 

 a position on the surface film, with head downward. In the 

 rejecting reaction of Stoichactis, described on p. 451, we have 

 clearly a reaction with reference to gravity, though one which even 

 the most sanguine could hardly denominate a fixed tropism. The 

 situation " waste - matter - on - the - disk - not - removed - by - the - 

 first- (usual) -reaction" is responded to by taking such a position 

 with reference to gravity as results in removing the waste; then 

 the reaction to gravity ceases. This is somewhat analogous to the 

 reaction to gravity described by Bohn (1903) in the hermit crab. 

 While investigating a shell which it may adopt as a home if fitting, 

 this animal takes up a certain position with reference to gravity — 

 namely, with the body on the steepest slope of the shell, and head 

 downward; it then turns the shell over and ceases to react with 

 reference to gravity. Of a different but equally significant char- 

 acter are the variations shown in the reactions to gravity by the 

 low acelous flatworm Convoluta, as described by Bohn ('03b) 

 and Gamble and Keeble ('03). Under conditions that are favor- 

 able Convoluta remains on the surface of the sand. But when the 

 sun becomes hot, or when the tide rises, so that the animal is 

 likely to be washed away, it becomes "positively geotropic," going 

 downward in the sand, where it is protected. When the tide falls 

 again Convoluta becomes "negatively geotropic," thus reaching 

 the surface of the sand, where it obtains food and carries on its 

 usual activities. These alternations of reaction become a fixed 

 habit with Convoluta, so that when removed to an aquarium it still 

 goes downward at high tide, upward at low tide, though the con- 

 ditions surrounding it remain constant; it may thus be used for a 

 time as an in-door tide indicator. Gradually, however, when 



