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



facts is therefore not so great. Although the number of deep-sea species which hav( 

 well-developed eyes is so large, they nearly all (all except three) belong to the two allied 

 genera Arcturus and Astacilla, which thus form almost the only exception to the general 

 statement that the deep-sea Isopoda are blind. It must not be forgotten that certain 

 genera, e.g., Pleiirogonium, which are confined to shallow water arc blind. 



It is extremely difficult to conceive of any reason for this great difference. Why 

 should the species of one genus retain their eyes after migrating into the deeper waters 

 while the species of another genus lose them ? Possibly the explanation of these 

 anomalies is to be sought for in considering the length of time that has elapsed since the 

 migration of the different species into the abyssal regions of the ocean. 



It will be, I hope, apparent to any one who has studied that portion of this Report 

 which deals with the description of species, that the peculiar deep-sea genera are will 

 marked as such, and it may be supposed, therefore, that they arc old inhabitants of the 

 deeper waters, consequently they have had time to lose their eyes ; on the other hand, 

 the deep-sea Arcturi are not widely different from their shallow-water allies, and perhaps 

 therefore have only recently become a part of the abyssal fauna ; these two assumptions 

 appear to me justifiable, but it is, on the other hand, evidently a pure assumption to 

 suppose that the deep-sea species of Janira, Scrolls, &c, have been just sufficiently long 

 inhabitants of the deep for the eyes to have become rudimentary or to have disappeared 



It is commonly stated that the deep-sea fauna comprises many species which are 

 larger than their shallow-water representatives. This is certainly the case with the 

 Isopoda. I need hardly allude to the appropriately-named Bathynomus giganteus,* "which 

 is by far the largest species of the order at present known ; there are no other instances 

 of deep-sea Isopoda which attain to so colossal a size, but in many families the deep-sea 

 forms are decidedly larger than the shallow-water. Among the AsellidsB, Stenetrium 

 haswetti is larger than any of the three shallow-water species of this genus ; Iolanthe is a 

 large Isopod compared with the size which other Asellidae reach ; the deep-sea species 

 of Eurycope are in many cases much larger than the species known to inhabit the shallow 

 w r ater, and I have already referred to the large size of two at least of the deep-sea species 

 of the genus Serolis. Many of the deep-sea Arcturi are of large size, but, on the other 

 hand, certain shallow-water forms, such as Arcturus haffini and Arcturus furcatus are 

 fully as large ; it must be noted, however, that both these species range into deep water. 

 Two deep-sea species of Ischnosoma, viz., Ischnosoma bacillus and Ischnosoma hacilloidcs, 

 are very much larger than the single species, Ischnosoma bispinosurn, which is known 

 from shallow water. 



A very noticeable feature in the organisation of the deep-sea Isopoda is the extremely 

 great development of spines upon the body ; this character is found in very many of the 

 different families which inhabit the deeper waters. The deep-sea Arcturi are more 



1 See p. 152. 



