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



comparatively small ; (2) that, as has already been pointed out, the deep-sea are in all 

 cases distinct from the shallow-water species ; (3) that the deep-sea species show certain 

 peculiarities, notably in the structure of the eyes, which, as has already been described 

 (p. 20 et seq.), are either entirely absent (Serolis antarctica) or, if present, show great 

 evidence of functional degeneration ; none of the deep-sea species possess well-developed 

 eyes. To compensate for the want of eyes, there is a great development of sensory hairs 

 on certain of the appendages; the males of Serolis necera, Sei'olis hivmleyana, and Semlifi 

 gracilis have, upon the third, fourth, and fifth joints of the third thoracic appendages, 

 tufts of sensory hairs, which have already been described (jjp. 55, 59, 62 ; PL IV. fig. 6 ; 

 PL V. fig. 8). This structural feature is not, however, peculiar to the deep-sea species, 

 inasmuch as it is also found in Serolis paradoxa ; but since it occurs in three out of the 

 four, and Serolis i^aradoxa is the only shallow-water species in which I have noticed it, 

 it may be considered as characteristic. Serolis antarctica does not agree with the other 

 deep-sea species in this respect, but the first pair of antennfe are furnished with a larger 

 number of sensory filaments than is usual — two upon each joint of the filament ; several 

 shallow-water species, however, Serolis pallida, e.g., and Serolis convexa, present the same 

 character. 



In two of the deep-sea species, Serolis hromleyana and Serolis necera, the genus 

 attains to its greatest size, and these are indeed among the largest of the Isopoda ; 

 as a general rule the deep-sea representatives of the Isopoda are not distinguishable 

 from their shallow-water aUies by their greater size ; there are exceptions to this rule, 

 notably in the case of Bathynomus, a deep-sea genus recently described by Milne-Edwards, 

 which is no less than 9 inches long; and the Challenger collection contains a specimen of 

 another Isopod belonging to the same family Cymothoadse, which is also of considerable size. 

 The elongated and spine-like epimera of Serolis newra and Serolis hromleyana, and also, 

 though to a less extent, of Serolis gracilis, are unlike anything that is met with in the 

 representatives of the genus from shallow water, where the epimera are always moderately 

 developed in comparison. In all the deep-sea species, without exception, the ambulatory 

 limbs are furnished with comparatively few spines, which are generally soft and delicate, 

 The strong sword-like and serrated spines so commonly found in the shaUow-water 

 members of the group are either completely or partially absent ; a very general character, 

 inasmuch as it is found in two out of the four deep-sea species (Serolis necera and 

 Serolis gracilis), is the presence, upon the ambulatory limbs, of plumose hairs similar to 

 those which are found upon the abdominal appendages; in Serolis necera especially are 

 these plumose hairs developed in great abundance. They have been more particularly 

 described above, on pp. 55, 56. 



The maxiUipedes in aU the deep-sea species possess a short tubercle on the inner 

 side of the middle joint of the palp, which may represent some kind of sense organ, 

 though the hairs wth which it is thickly covered are in no way different from the hairs 



