32 Papers from the Marine Biological Laboratory at Tortugas. 



been ascribed to Pagnrus by Bohn (1903). Ocypodas are often seen care- 

 fully tamping down the mound in front of their burrows by means of the 

 flat outer side of the large chela, making it round and smooth. Occasionally 

 when there is a small hollow in the surface they will bring sand and fill it 

 up. Sometimes I have seen individuals, which had apparently lost their 

 bearings in the hasty return to their burrows, behave as though they were 

 testing the surface of little mounds which were not situated in front of 

 burrows. 



Several factors contribute in bringing about the accurate return of these 

 crabs to their homes. They are undoubtedly guided by dififerences in the 

 lighting of surfaces, by tactile stimuli, by differences in muscular effort, and 

 by stimulation of the equilibrating organs resulting from a tilting of the body. 



BEHAVIOR TOWARD WATER. 



While Ocypoda lives most of the time on land, it is absolutely necessary 

 for it to go into the water occasionally. I am unable to say how long it 

 stays in the ocean during tlie breeding season, but at other times, so far as 

 my observations go, it remains there only a short time unless forced to do 

 so by some enemy that prevents its return. 



Adult individuals may spend hours in their burrows without going near 

 the sea, but the young ones seem to be much more dependent on a fresh 

 supply of water. This may account for the fact that their burrows are built 

 closer to the water-line than those of the adults. 



When an ocypoda is startled or disturbed while far away from its bur- 

 row it is very apt to run into the water, but even then it probably never 

 goes out more than 4 or 5 feet from the shore. Ocypoda does not swim, 

 but crawls along the bottom and is washed back and forth by the surf. 



When undisturbed, Ocypoda goes down to the ocean now and then in 

 order to moisten its gills with fresh sea-water ; but at these times the indi- 

 viduals do not enter the water ; they settle down about 6 or 8 inches from 

 the water-line formed by medium-sized waves, with the ambulatory ap- 

 pendages of one side presented to the ocean and those of the other side 

 firmly embedded in the sand. In this position they wait until an extra high 

 wave washes over them and then return to the higher parts of the beach. 

 Sometimes, after remaining in a place for a considerable period without 

 being wetted by a wave, the crab will change its position to one closer to the 

 water. 



While Ocypoda does not live in the ocean much of the time, it is able to 

 do so, as is shown by the following experiments : A large adult male was 

 put in an aquarium filled with sea-water and left there for 6 hours. It was 

 apparently in good condition when liberated at the end of this time. An- 

 other specimen left in the aquarium for 24 hours was also active when 

 released. Other specimens placed in fresh water lived only for 5 hours and 

 made frantic attempts to escape. 



