10 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvn. 



these cases this embryonic or larval character is permanently retained 

 in the adult condition. 



Many of the parasitic Isopods, such as the Epicaridea, differ mark- 

 edly in their structure from the free forms of other Isopods. For 

 example, the females of the famil}^ Dajidte have but five pairs of tho- 

 racic feet, crowded together around the oral area, and BrancJiJopJiri/xus 

 Cauller}^, a recently described genus of the family, has but four pairs 

 of legs present in the adult female. Phryxus abdominall.s Kr0yer, an 

 Epicarid species, has all the thoracic legs present on one side of the 

 body in the female, while on the other side they have all disappeared 

 with the exception of the first. 



There is no trace of thoracic feet in the females of the Cryptoniscidse, 

 parasitic on the Amphipoda and other Isopoda, the Ostracoda and the 

 Cirripedia, especially a parasitic family of Cirripedes, the Rhizo- 

 cephala. 



The males of the Entoniscidse have the seventh thoracic segment 

 without appendages, the other six segments sometimes with rudi- 

 mentary feet; the female also has rudimentary feet. 



The legs are composed of seven joints. Beginning at the proximal 

 end, or their point of attachment with the thorax, these joints are: 

 The coxa or coxopodite, the basis or basipodite, the ischium or ischi- 

 opodite, the merus or meropodite, the carpus or carpopodite, the 

 propodus or propodite, and the dactylus or dactylopodite. The 

 dactylus is sometimes furnished with an ungulus, which may be uni-, 

 bi-, or tri-fid. The females in the genus Kepon Duvernoy have the 

 feet ending in inflated joints without unguli. 



Variation in the number of joints is found among the Gnathiidse 

 where the first gnathopods are only two- jointed with the males, are 

 "opercular, the first joint being a large pyriform plate, fringed with 

 setffi on the convex inner margin and containing three semitransparent 

 calcareous plates, supposed to indicate the same number of original 

 joints. "« In EucognatJiia gigas (Beddard) the first gnathopods in the 

 male are only six-jointed. Chsetella Dana has the sixth and seventh 

 pairs of legs terminating in an extremity composed of numerous 

 joints. 



In many Isopods (Oniscoidea) the legs or periopods are all similar 

 in shape and size and are ambulatory in character. Difference of 

 structure is to be found, however, in other groups. The Tanaidae, 

 for example, have the first pair of legs or gnathopods transformed 

 into chelipeds. The chelae of the males are much stronger and more 

 robust than those of the females, and in some genera, as Lejytochelia 

 Dana, they are greatly elongated in many of the species. In the 

 Apseudidffi the first and second gnathopods are modified, the first pair 



aStebbing, History of Crustacea, 1893, p. 336. 



