i6 Papers from the Marine Biological Laboratory at Tortugas. 



the body. Who has not been tempted to say, when he walks along the 

 beach and sees these crabs in the far distance scampering in haste toward 

 their burrows, that they have a keen sense of vision? Thus far, however, 

 we have no proof that Crustacea have vision that definite pictures, such as 

 we know are formed in the eyes of the higher vertebrates, occur in the eyes 

 of these invertebrates. 



The stalked eyes of Ocypoda are capable of considerable movement. 

 They may be dropped laterally into grooves under the anterior edge of the 

 carapace, where they are quite well protected. Besides this lateral move- 

 ment I have noticed individuals move the eye-stalks backwards and for- 

 wards while sitting perfectly still on a level surface. It is of interest to note 

 that individuals when starting down their burrows (they always go down 

 sideways) drop the foremost eye-stalk into the groove under the carapace, 

 leaving the hindmost one erect. The advantage of this procedure can easily 

 be seen. When the eyes are erect the angle of possible movement toward the 

 sagittal plane of the body is very small, so that in going down the burrow, 

 if the eye-stalk were kept erect there would be danger of its being forcibly 

 bent or broken off by striking against the walls of the burrow. The hind- 

 most eye-stalk, however, does not run this danger. Furthermore, it is not 

 probable that the crab receives any very definite light stimuli through the 

 eye on the side presented to the dark end of the burrow, while the other 

 eye undoubtedly serves as a means of detecting any lessening of the intensity 

 of light at the opening of the burrow which might be caused by the approach 

 of an enemy. 



In Ocypoda the part of the eye sensitive to light extends over quite a 

 large surface and covers much of the distal end of tlie stalk. It is not 

 evenly distributed; the largest surface exposed 

 is on the anterior side; the next largest is on 

 the outer sides; the next on the posterior side 

 and the smallest is on the dorsal surface (fig. 

 7). This distribution of the sensitive surface 

 corresponds with the attitudes commonly as- 

 sumed when a crab is watching. In the majority 

 of cases it presents either the entire surface or 

 the side of the body toward the object in ques- 

 tion ; less frequently the posterior surface. The 

 dorsal surface of the eye which has the smallest 



sensitive surface is no doubt used the least, err c r, 



J . Fig. 7. Surface of eye of Ocy- 



since no enemies approach from above and since p^j g^ anterior view ; b, 



the food is on the ground. As we shall see posterior view; c, outer side; 



, , , , li. inner side; e, dorsal side. 



below, this crab seldom reacts to the movements 

 of objects directly above it. 



f 



