154 THE VOYAGE OF H.M.S. CHALLENGER. 



and fifth pairs of abdominal limbs are respiratory ; in the fifth pair both the inner and 

 outer ramus are thin-walled, delicate, much folded lanielhe, in the fourth pair it is only the 

 endopodite of the limb which has this peculiar structure, the exopodite being a flat hard 

 plate which covers over the subjacent lamellae and serves as a protective operculum. In 

 Cymodocea dbyssorum, however, the fourth pair of appendages exactly resembles the 

 fifth pair; there is no operculum, both exopodite and endopodite are folded gill-plates. 

 This feature, although it may be correlated with the habitat of the Crustacean, is not so 

 noteworthy, since it occurs in the shallow-water genus Amphoroidea according to Milne- 

 Edwards ; l and I have been able myself to verify his statements by an examination 

 of Amphoroidea falcifer, a New Zealand form, which I have had the opportunity of 

 studying through the kindness of Mr. G. M. Thomson. 



Possibly the delicate integument of certain deep-sea Munnopsids is a modifi- 

 cation brought about by a similar need for increased respiration. In other Isopoda 

 I can detect no obvious evidence which would point to a need for increased respiratory 

 power. This is one of the most puzzling facts about the deep-sea Isopodan fauna, 

 that whereas some forms, such as Anuropus and Bathynomus, show an evident 

 modification in relation to their habitat, others show no traces of any such modi- 

 fication and are indistinguishable in these respects from shallow-water forms. As in 

 the case of the presence or absence of eyes in the deep-sea Isopoda discussed below 

 (p. 163) it may be that these special modifications indicate a remote period of immi- 

 gration into deep water. 



Anuropus branchiatus, F. E. Beddard (PI. VII. figs. 1-5). 



Anuropus branchiatus, F. E. Beddard, Proe. Zool. Soc. Lond., 1886, pt. i, p. 113. 



The single specimen of this species measures 70 mm. in length. 



The body is extremely convex, more particularly in the thoracic region. 



The head is small and rounded, with a short median rostrum ; there is no trace of 

 any eyes ; the anterior region of the head is deeply grooved for the insertion of the 

 autennary organs, the ventral margin of the head is ridged below, and in the middle line 

 it is prolonged into an upwardly directed process which exactly corresponds in direction 

 to the rostrum ; the two do not, however, meet, but are separated by a space about equal 

 to the basal joint of the antennules. 



The thoracic segments increase progressively in length from before backwards up to 

 the sixth ; the seventh is a little shorter than the sixth. 



All the segments with the exception of the first are furnished with separate epimera. 



The abdomen is distinctly narrower than the thorax, the first five segments are 

 subequal; their lateral margins are rounded and overlap, the antero-lateral region of the 



1 Hist. Nat. Crust, t. iii. p. 222, pi. xxxii. fig. 9. 



