CRUSTACEA — WIIITELEGGE. 



215 



Total len 



gth of body 



.. 6-2r, 



nun 



Length of cephalon .. . 



.. 1-2 







, peraeun ... 



.. .3-S 







, pleon 



.. 1-25 







, first antenna 



.. 1-5 







, second antenna ... 



.. 0-4 







, palp of mandible 



.. 0-45 







, first chelipede ... 



.. 2-5 







, second leg 



.. 2-7 







, third leg ... 



.. 1-25 







, seventh leg 



.. 0-8 







, first pleopods, with the setse 



.. 2-0 







, third „ 



.. 1-5 





In habit as well as in structure this species is equally remark- 

 able, and affords a striking instance of adaptation to a mode of 

 life quite different from that of other members of its tribe. It is 

 distinctly paguroid in habit, living in small univalve shells and 

 in company with young hermit crabs. The resemblance to the 

 latter is so complete that it requires close inspection to separate 

 the two forms. This mimicry of external appearance and the 

 similarity of the habitat are of extreme interest from a homo- 

 plastic point of view, showing how habit and environment tend 

 to induce the development or degradation of a number of import- 

 ant homogenetic characters in widely separated organisms. The 

 paguroid features presented by this singular Isopod may be 

 •enumerated as follows : — The body is strongly curved and adapted 

 to the shape of the columella of the shell, as are also the cheli- 

 pedes, neither of which can be straightened without breaking. 

 The hands are distinctly curved and very similar to those of the 

 young of many species of Clihanarkis, the fingers moving in a 

 horizontal plane. The second pair of legs is long and ambulatory, 

 not dilated and fossorial as in its nearest allies. The succeeding 

 five pairs of legs are greatly reduced in size and the inferior 

 surfaces of the fourth, fifth and sixth joints are clothed with 

 numerous short, rasp-like teeth, identical in form, origin and 

 structure with the pad or rasp present on the reduced legs of the 

 pagurids. The pleon is symmetrical, but together with its 

 appendages exhibits evidences of degradation. The segments do 

 not display any points of articulation ; inferiorly they are very 

 short, interrupted and only partially calcified. The pleopoda are 

 frequently absent; when present they are small and rudimental. 

 The uropods with their rami do not exceed the telson in length. 



