214 TRANSACTIONS OF THE [maR. 8,. 



ON NEREIDS COMMENSAL WITH HERMIT CRABS. 



By N. R. Harrington. 



The annelids are not very generally addicted to commensal or 

 parasitic habits. Many Polynoids, however, live on the houses 

 or bodies of their neighbors and a species of Polydoi^a has been 

 described * which tunnels for itself a house in snail shells in- 

 habited by hermit crabs. This latter host has been known for 

 several years to harbor a western European species, JVereis 

 fucata, Sav. although it has only recently been shown f that this 

 commensal mode of life is correlated with structural modifica- 

 tions in the body of the worm. 



The purpose of this paper is to give a comparative description, 

 of two almost similar commensal forms from the Pacific coast. 

 Although it has been necessar}^ to make a new species of the 

 Nereid, and although the hermit crab is regarded as a modifica- 

 tion of the type form, \ the general resembance of the commen- 

 salists of the Old World to their respective representatives in 

 the New, gives evidence of a common origin of this commensal 

 habit. 



The agency of ocean currents in transporting long persisting 

 larvfe has generally been accepted as accounting for widely 

 separated areas of distribution. § In the distribution of certain 

 commensal forms this factor might be eliminated by different 

 larval periods in two mutually dependent species. If, as we as- 

 sume in the present case, the host is cosmopolitan, ocean cur- 

 rents might still be a factor in the extension of the species of 

 the guest. 



Aside from a comparative study of the commensal habit, this 

 paper deals with the sexual phases of the Nereid under discus- 

 sion and some possible causes or factors in the sexual metamor- 

 phism. 



Many valuable suggestions and much useful information were 

 given me by Prof. A. E. Yerrill, of Yale University. The ex- 

 cellent drawing of the head of Nereis cyclurus was made by 

 my friend, Mr. J. H. McGregor. I am indebted to Mr. A. Ryd- 



* Andrews in American Naturalist for 1892. 



t Wirt^n Cm en hos Eremitkraftor lefv. Annelid, in Kon. Sven. Veten. Akad. Hand- 

 ling., Stockholm, 1888, Bi. 14. 



X It agrees most closely with Eupagurus armalux, Dana, as I am informed by Mr. 

 W. T. Colman, who has worked up the Crustacea of the expedition. At first erron- 

 eously thought it E. alaskensis. 



l Andrews, Johns Hopkins Giro. x. p. 96. 



