56 NATURAL SCIENCE. July, 



the authors of the Reports on Isopoda and Cirripedia, as well as a short 

 account of the Ostracoda by our veteran authority on that group. 



In the number of new species and genera, the " Challenger " 

 collection of Isopoda excels any yet formed. In certain genera, 

 notably Anturus and Serolis, the number of species collected double 

 the existing lists. In the Asellidae a great number of new shallow- 

 water species were collected round the shores of Kerguelen, while the 

 deep sea yielded numerous interesting forms. 



Apart from additions to our faunal lists, the chief facts of interest 

 derived from the study of the " Challenger " Isopoda, concern the 

 modifications induced by life at great depths in the ocean. Many 

 deep-sea animals are totally blind ; yet many species found in the 

 deepest hollows of the ocean, appear to have perfectly normal eyes. 

 These discrepancies were partly accounted for by the theory of 

 abyssal light. The histological study of the eyes of certain deep-sea 

 Isopoda, particularly of Serolis and Arcturiis, shows, however, that the 

 appearance of well-developed eyes was often deceptive. Anyone, 

 before having recourse to the microtome, would assert that the deep- 

 sea Serolis necera was as keen-eyed as any species of the genus. Yet 

 sections through the eye show that it is in a condition of degeneration ; 

 apart from the faceted cornea there is but little of recognisable eye- 

 structure left. In S. hromleyana the eyes are well marked but entirely 

 devoid of pigment ; no trace of optic tissue could be found by 

 microscopical investigation. It seems, therefore, as if the external 

 and less important portions of the optic organs were the last to go. 

 Several species of Arcturns indicate similar degradation in the eyes; 

 in some the crystalline lens had increased in size, become opaque and 

 lost its clear-cut outline. It appears, therefore, that to explain the 

 occasional persistence of well-developed eyes in this group, there is 

 no need of any theory of abyssal light ; it is more likely that the 

 state of preservation of the eyes is an index of the length of time that 

 the species in question has been an inhabitant of the deeper waters. 

 The " Challenger " material also enabled me to describe a new type 

 of eye, confined to the Serolidae, and to the closely allied Cymothoidae, 

 and recently found in other species of Serolis by Watase. 



Modifications due to the scarcity of oxygen at great depths were 

 found in two remarkable new types. In all Isopoda (except certain 

 parasitic forms) the last pair of abdominal appendages are swimming 

 feet, not modified for breathing as are the pairs in front ; but in 

 Anuropus, a genus of Cymothoidae, these appendages also entirely 

 resemble the branchial abdominal appendages which lie in front of 

 them. In Munnopsis pellucida the skin, instead of being thick and 

 strongly calcified, as is the case with all other Isopoda, is thin and 

 transparent ; this, I imagine, would facilitate the absorption of greater 

 quantites of oxygen from the water. 



In the deep-sea forms of Isopoda as of other Crustacea, spines are 

 often richly developed. In Serolis bromleyana (PI. xiii., Fig. 2), S. nea^ra and 



