REPORT ON THE ISOPODA. 165 



Station 281. Arcturus abyssicola. 



Station 302. Acanthocope spinicauda, Ischosoma bacilloides. 



Station 318. Serolis nesera. 



( Station 320. Serolis nesera, Stenetrium haswelli, Arcturus americana. 



1 Station 323. Neotanais americanus. 



It appears from the above list that the deep-sea Isopoda are distributed very 

 unevenly over the floor of the ocean, and that there are long stretches of ocean where no 

 species at all were found ; these are the whole of the Central and Southern Atlantic, and 

 the Central and Western Pacific. 



I have bracketed together those Stations which are only separated by a very short 

 interval, and this will show that, in most instances where Isopoda were met with, they 

 were represented by a considerable variety of forms. More particularly to be noticed 

 are Stations 146 and 147, Stations 168 and 169, Station 153, Station 158, Station 218, 

 Stations 320 and 323, and Station 246. 



The accompanying map illustrates the distribution of the deep-sea Isopoda^ 



SPECIAL PECULIAEITIES OF THE DEEP-SEA ISOPODA. 



It has been long known that the deep-sea Crustacea, as well as other deep-sea animals, 

 are commonly blind, but the puzzling fact that this is by no means universally the case 

 has exercised the ingenuity of naturalists ; the well-known theory of abyssal light has 

 been brought forward as an explanation of the persistence of eyes in certain forms. 



Among the Isopoda thirty-four of the deep-sea species are totally blind, and three 

 others, viz., one species of Eurycope and two of Ischnosoma, only represented by 

 fragments, may in all probability be added to this list ; in four species, viz. , three species 

 of Serolis 1 and Cymodocea abyssorum, the eyes are evidently degenerate ; in eighteen 

 species, on the other hand, there are well-developed eyes. 



The significance of these facts, however, is not apparent from the statistics ; they will 

 require to be examined more closely. In the first place, it is noteworthy that of the 

 eleven peculiar deep-sea genera only two, viz., Acanthomunna and BatJiynomus, have 

 eyes ; of the remaining species, seven belong to genera of which the shallow-water repre- 

 sentatives, so far as is known at present, invariably possess eyes ; 2 these two series of 

 facts are evidently of considerable importance. 



On the other hand, the remaining eighteen, included in the genera Munnopsis, 

 Eurycope, Ischnosoma, Typhlotanais, and Cryptocope, belong to genera which are 

 invariably blind, even when occurring in cpiite shallow water ; the significance of these 



1 See Part I. of my Report, Zool. Chall. Exp., pt. xxxiii. 



2 This includes those species with imperfect eyes, evidently on the way to disappearance. 



