426 NATURAL. SCIENCE. JuNE, 
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 Pterotvachea, has a pair of ‘‘ otocysts”’ 
