6 Papers from the Marine Biological Laboratory at Tortugas. 



exterior and sometimes it does not. In the first case I have found it used 

 only rarely for entrance and exit. As a rule the opening of the branch 

 passage is much smaller than that of the burrow proper and usually there 

 are no tracks leading from the opening of the branch passage, while the sand 

 around the entrance to the burrow is covered with them. These facts seem 

 to indicate that the branch passage is not made for habitual entrance and exit. 



It occurred to me that this second opening might be a means of venti- 

 lating the burrow, thus making it cooler. There are absolutely no facts to 

 support this theory, especially since it is found that the deep burrows, which 

 would need ventilation if any did, were without these branching passages. 



Some light was thrown on the question when an ocypoda was observed 

 digging its burrow. After it had dug for a certain distance, bringing up the 

 sand at regular intervals, it remained down in the burrow for some little 

 time and then suddenly there occurred a disturbance of the surface of sand 

 in about the region where the opening to the second passageway should be. 

 The sand began to sink in a little, but no opening was formed, and it re- 

 mained in this condition until I dug up the burrow, when it was found that 

 the crab had made a branch passage, starting from the burrow and working 

 up almost to the surface without quite breaking through. Such a procedure 

 was witnessed on more thnn one occasion, and each time the branch passage 

 did not quite break through to the surface. 



After observing this it occurred to me that the passage might be used in 

 time of need to escape from enemies, and I soon witnessed a sight which con- 

 vinced me that such was the case. One ocypoda was in possession of a 

 burrow with a branching passage, when another appeared at the opening of 

 the burrow. The owner immediately went down and the intruder followed 

 for a short distance and then returned to the opening, followed by the owner. 

 This was repeated several times, the intruder going down farther each time, 

 when suddenly the owner climbed up into the branching passage, broke 

 through the surface and scampered away, leaving the intruder in possession. 

 The two crabs were evidently contending for the possession of the burrow, 

 and it seems probable that when the owner was unable to keep the intruder 

 out it escaped by the only means possible in order to prevent injury to itself 

 when driven to the end of the burrow. 



This same procedure was witnessed several times, but occasionally it was 

 varied by the owner going around to the opening of the burrow and driving 

 the intruder up through the branching passage instead of running away 

 after its escape. Frequently this chase in through the burrow opening and 

 out through the branched passage would be continued for some time, making 

 a very ludicrous sight for the observer. 



The behavior just mentioned, together with the facts that the opening of 

 the branching passage is usually not made at the same time the passage is, 

 and that there arc but few, if any, tracks around the opening, leads me to 



