426 NATURAL SCIENCE. Juxe. 



where the movement of the water is great enough to tilt the animal 

 into an oblique position. 



In what zone this will happen depends upon conditions too com- 

 plex for consideration here, but the direction in which the tilting will 

 occur is a matter worth considering, and one which presents no great 

 difficulty. 



Each layer of water becomes thinner when under a trough of the 

 surface-wave, and thicker when under a crest. The ratio of the 

 velocities of two layers is constant so long as the surface disturbance 

 remains unchanged. Hence it is obvious that as the jelly-fish extends 

 through more layers when under a trough (where the layers are 

 thinnest) than when under a crest, the difference in the rates of 

 movement of the uppermost and lowermost of the layers occupied by 

 the jelly-fish is greatest when under the trough, that is, when the 

 movement is opposite in direction to that of the waves. Hence the 

 alternate tilts in opposite directions will not be equal, and the total 

 effect will be that the animal is turned with its aboral (or "upper") 

 surface " up-storm," that is, towards the direction from which the 

 waves are coming. 



The rhythmic contractions continuing, the animal will swim 

 upwards and in a direction opposite to that of the waves. As it does 

 so it comes into new zones, each moving more rapidly than the 

 previous ones, till at length a zone is reached where the changes in 

 velocity are so great that the slight elasticity of the stalk of the tenta- 

 culocyst is no longer sufficient to keep the weighted tip from lagging 

 so far behind the pouch as to touch its walls. At the moment when 

 it touches, a stimulus is produced, which causes a more energetic 

 contraction of the bell, especially in the adjacent portion of the bell. 

 The tentaculocysts first so affected will, of course, be those exposed 

 to most rapid changes of velocity, that is, those in the edge which is 

 turned upwards. The vigorous contraction of this portion of the bell 

 in advance of the contraction of the rest will turn the animal, with its 

 aboral face, first, directly towards the point of the compass from 

 which the waves proceed, and then gradually downwards, the swim- 

 ming movements taking the animal to a greater depth, and in the 

 direction opposite to that in which the waves travel. 



When a deeper zone is reached, where the movement is too slight 

 to bring the lithite, or weighted end of the tentacular portion of the 

 tentaculocyst, into contact with the sides of the pouch, the greater 

 specific gravity of the oral portion of the animal again turns the aboral 

 surface somewhat upwards, and the animal again approaches the 

 surface. 



The animal is thus automatically steered so as to be kept always 

 in the uppermost zone of safety — a zone of maximum food-supply ; 

 and this explanation involves no assumption as to consciousness, or 

 sensation, or judgment on the part of the jelly-fish. 



A pelagic mollusc, such as Ptcrotracliea, has a pair of " otocysts " 



