CRTS'! ACKANS IN ( I I.KK )!' I I'.K IS TUBES. 113 



taken up life in Chcctoptents tul>rs. While this in no way dis- 

 proves Shelford's statement as a general proposition, the three 

 factors would, in the present case, come in the following order; 

 the most important first: (i) protection, (2) food, (3) breeding. 

 Of course, if suitable conditions for breeding were absent the 

 crustaceans would become extinct; but given the racial habit of 

 carrying eggs and the abundance of pelagic microorganisms in 

 littoral waters for food, reproduction takes care of itself, is 

 fostered, in fact, and its products are unusually abundant. 



Caiman ('11, p. 217), in speaking of the association of Pin- 

 notheres with molluscs, says, "The case is, indeed, an example of 

 the difficulty of defining these two terms (commensal, parasite). 

 At all events the Pinnotherid crabs show one of the character- 

 istics of parasites in being to some extent degenerate in their 

 structure. The carapace and the rest of the exoskeleton, no 

 longer needed for protection, have become soft and membranous, 

 and the eyes and antennules, the chief organs of sense, are very 

 minute. As in many parasites, also, the eggs are very numerous, 

 and the abdomen is very broad and deeply hollowed out for their 

 "reception." To this list I may also add that Pinnixas fourth 

 and fifth legs are used as hook-like claws for holding on, like the 

 organs of fixation in many parasites. 



There is no question but that the two commensal crustaceans 

 discussed gain from their association with Chcetoptenis, but it is 

 doubtful if the worm is benefited. 



BIBLIOGRAPHY. 

 Caiman, W. T. 



'n The Life of Crustacea. New York, xvii +289-99. 

 Enders, H. E. 



'05 Notes on the Commensals found in the Tubes of Chcetopterus pergamen- 



taceus. Amer. Nat., 39, pp. 37~4O. 

 '09 A Study of the Life-history and Habits of Chaloplerus variopedalus Renier 



et Claparede. Jn. Morphol., 20, pp. 47Q-S3 2 . pls - l ~3- 

 Mast, S. O. 



'ii Light and the Behavior of Organisms. New York, xi +410 pp. 

 Osborn, R. C. 

 '12 The Bryozoa of the Woods Hole Region. Bull. U. S. Bur. Fish., 30, pp. 



203-266, Pis. 18-31. 

 Rathbun, M. J. 



'05 Fauna of New England. 5. List of the Crustacea. Boston Soc. Nat. 

 Hist., Occ. Papers. 7, pp. 1-117. 



