Habits, Reactions, and Associations in Ocypoda arcnaria. 15 



hardly be explained in any other way, and, as we shall see below, the move- 

 ment of objects has an important influence on the behavior of Ocypoda. 



Experiment H. — A piece of decomposing fish was placed in a hole at 

 the center of the square, the hole was filled up and the surface then smoothed 

 off. The observer then left, but after 30 minutes returned and found that a 

 crab had dug the fish out and carried it away. Considerable precaution was 

 taken to prevent the surface sand from touching the fish or its juices, so 

 that the indications are that the ocypoda found the food through the 

 stimulus of odor, the eyes not aiding in its detection. 



Experiment I. — In this experiment a piece of decomposing fish was 

 fastened to the end of a stick, which was then stuck in the sand, so that the 

 meat was 60 cm. from the ground. A crab ran up to the stick, pinched it, 

 and tried to reach higher than usual with its chelae, but was unsuccessful in 

 what appeared to be an attempt to get the fish. After this the crab returned 

 to its burrow and did not go back again. 



Experiment J. — -The conditions were the same as those of Experiment I, 

 except that the stick was pushed down into the sand until the fish was 4 cm. 

 from the surface of the ground. Flies were thickly clustered about the 

 food when a crab approached. This individual did not reach up and take 

 the fish, but made an attack by jumping quickly at the food, striking it with 

 its claws and jumping away, after which it moved off quite a distance. 

 Then the ocypoda repeated this same behavior several times, until the flies 

 were scared away, when finally it reached up, standing on the last two pairs 

 of walking appendages, and began to eat. The stick was then raised again 

 until the fish was 50 cm. from the sand, after which the same crab returned, 

 but did not reach up. The fish was soon abandoned. Several times during 

 the afternoon a small crab attempted to climb up, i. e., put its claws against 

 the side of the stick and raised itself, but was never successful in reaching 

 the food. 



While it can hardly be claimed that any of the above experiments fur- 

 nish conclusive proof that Ocypoda is stimulated by odor, experiments 5 

 and H point strongly in that direction. The behavior in most of the rest 

 of the experiments may be explained by any one of the following hy- 

 potheses: (i) That Ocypoda reacts to odor. (2) That opaque objects are 

 distinguished as a result of the diflierence between the intensity of light on 

 the object and the region surrounding. (3) That Ocypoda actually has 

 vision. (4) That any two or all of these factors determine the behavior. 



As we shall see later, the author was unable to obtain any evidence that 

 Ocypoda has vision. It might be held by those who do not admit that Crus- 

 tacea see with their eyes or react to odors, that Ocypoda approaches food and 

 other objects merely by chance, but those who have observed this interesting 

 crab can not for one moment believe this to be the case. 



EYES. 



Anyone who, for the first time, sees an ocypoda running over the white 

 sand is forcibly impressed by the prominent and relatively large stalked eyes 

 which, being almost black in color, form a striking contrast to the rest of 



