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34 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
compensation movement of the eyestalks of the normal crayfish through 
an angle of 10° to 18° ; and, further, that when the animal is rotated 
about its long axis blinding causes a diminution of 10% in the angle of 
compensation. His results therefore give a much more important place 
to vision in orientation than do the conclusions of Clark and myself. 
However, from the combined results of the experiments of Clark, Lyon, 
and myself, one cannot avoid the conclusion that, in the fiddler crab at 
least, the otocyst is by far the most important organ in equilibration ; 
next in order comes vision, and then muscular and tactile sense. 
3. Lquilibration of Animals normally without Otocysts. 
Virbius zostericola, a shrimp quite common at Wood’s Hole, Mass., 
does not possess otocysts. Observation and experiment brought out 
several interesting facts concerning it. In the first place, it is not a free- 
swimming form. Its normal habitat is on the eel grass, to which it 
clings in positions indifferent to the direction of gravity. When forced 
to swim, it does so in a very uncertain manner, with the dorsal side usu- 
ally uppermost, though this is a position of unstable equilibrium. If 
overturned artificially (and this is easily accomplished), it rights itself 
slowly and will cling to the first object it may chance to touch. Re- 
moved from its supporting blades of eel-grass, its unstable manner of 
swimming closely resembles that of shrimps in which the otocysts have 
been destroyed. If the eyestalks are painted with lampblack, and the 
animals so treated are placed in a large aquarium, and forced to swim, 
apparently all sense of direction and means of orientation are lost. 
4. The Effect of the Development of the Otocyst on the Equilibration of 
Lobster Larve. 
As has been shown in the morphological part of this paper, there is no 
otocyst in the newly hatched larva of either Palemonetes, the lobster, 
or the crab, nor is there a functional organ during the first three larval 
stages. It begins to invaginate only in the second larval stage, and it is 
merely a shallow cup-like depression in the third stage; not until the 
next moult do the sensory hairs and otoliths appear. 
When we examine the conditions as to equilibration and manner of 
swimming in the different larval stages, we find that in the first larva 
the body is not definitely oriented while swimming. Newly hatched 
lobsters are very unstable in their movements, often swim or come to rest 
upon their backs or sides, and show a tendency to roll from side to side 
while swimming. The animal swims by means of the exopods of the 
