234 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. Equilibration 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 Larvce. 



As has been shown in the morphological part of this paper, there is no 

 otocyst in the newly hatched larva of either Palaemonetes, 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 



