IO4 A. S. PEARSE. 



of the jar and crouched on the sand. None of the crustaceans 

 moved at once to the projecting tube tips, and sometimes they 

 even came in contact with a tube without reacting. After a 

 time, however, they would begin slowly exploring the surface of 

 the sand, and, if they then came in contact with a tube, usually 

 entered at once, whether a Chcetopterus was inside or not. Ap- 

 parently they found the tubes by accident. 



In regard to the ability of the commensals to leave the tube 

 Van Beneden ('76, p. 20) says, "On the coast of Brazil, my son 

 found two couples of crabs in the tube of a very long annelid, 

 narrow at the ends, and wide in the middle. The tube was too 

 small at the end to allow them to escape. These crustaceans had, 

 no doubt, penetrated thither before they had attained their full 

 size. Enders ('05, p. 39) also believes that, "Once in, the 

 crabs remain there and are later prevented through their own 

 growth from escaping. . . , When a worm in a tube dies the 

 crabs in the same tube die as a result of the failure of food and 

 properly aerated water." Some tubes may be too small to allow 

 the commensals to pass in and out, but there can be no doubt 

 that they enter and emerge again in some cases, as the two 

 following experiments show. 



Experiment i. On July 19 three Pinnixce, two males and a 

 female, were placed on the sand in the jar containing the tube 

 previously described, which contained a living Chcetopterus. 

 Next morning all had entered the tube; the Chcetopterus was 

 dead. At 2.00 p. m. two of the crabs had come out on the 

 surface of the sand, but one still remained in the tube. 



Experiment 2. After the last experiment the tube was 

 removed, thoroughly cleaned, and replaced in the jar with fresh 

 sand and sea water, but no Chcetopterus was placed in it. On 

 July 20, at 5.00 p. m., the two largest Pinnixce obtainable (male, 

 13 mm. wide; female, 12.5 mm.) were placed on the sand; 

 at 9.00 a. m. the next morning the male was in the tube, and at 

 12.30 p. m. the female had entered. On July 22, at 3.00 p. m., 

 a male (9 mm.) and a female (12.5 mm.) Polyonyx were also 

 placed in the jar. The male entered at 3.21 and the female at 

 3.40 p. m. All four crabs remained within the tube until July 24 

 at 4.45 p. m., when I observed that the water had become foul; 



