218 TRANSACTIONS OF THE |mAR. 8, 



might be found free-swimming. (2) No female epitocons form of 

 N. fucata (type form) is known. Is not the form of N. fucata 

 which lives in snail shells the female epitocous form? The 

 characters upon which Wiren has decided that the latter is a 

 different variety from the free living form are those peculiar to 

 epitocous formSj such as our type h or c. These characters are 

 (1) the modification of posterior end of body, and (2) enlarge- 

 ment of upper lobe of upper part of foot. Aside from the 

 physiological results from the commensal mode of life, variety 

 ji inquilina is not more Avidely different from the type atocous 

 form than are epitocous forms generall3^ 



The observations on N. Dumerillii demonstrate that sexual 

 garb and habits are not exactly correlated. Professor Verrill 

 has kindly shown me in his museum a series from Nereis to 

 Heteronereis forms illustrating all gradations in number and 

 nature of metamorphosed parapodia and, after the finding of a 

 Heteronereis type in N. cyclurus, the statement of Wiren does 

 not seem convincing, that an unchanged female N.fuc. ft inquilina, 

 necessarily presupposes (or excludes) a corresponding unchanged 

 male -3 inquilina. This commensal variety of N. fucata^ as we 

 now know it, rests ultimately on morphological divergences of 

 the female, incidental to a parasitic life. 



To sum up, commensal females, including immature adult and 

 metamorphosed types, are the phases of Nereis cyclurus now 

 known. The males are presumably free living. We venture 

 the suggestion that since no male N. fucata (3 inquilina and 

 no female epitocous type form of N. fucata are known, that the 

 variet}'^ /3 may be the usual epitocous female form of the latter. 



IV. Relation of Old and New World Commensal Nereids.. 



Owing to lack of material of the European species I have not 

 been able to make a careful comparison of the Crustacea in- 

 volved in this commensalism. The western American hermit- 

 crab, which is host to all large Nereids, agrees most closely with 

 Dana's species, armatus,* of the type Eupagurus hernhardus. 



A careful comparison of drawings with Wiren's exhaustive 

 description of N. fucata and a reexamination of the material 

 brings out the following ijeneral resemblances in the Nereids : 



(1) Those resulting from degeneration of posterior end of body. 



(2) Shape of antennje, palpi and jaws. (3) Characteristic large 

 dorsal lobes of notopodium. 



The differences are : (I) Arrangement of paragnathi. (2) Com- 

 plete difference in distribution of pigment. N. cyclurus has no- 



*Proc. Phil. Acad. N. S. 1051. 



