174 DE. C. CHILTON ON THE SUBTERRANEAN 



have to some extent been bridged over in the same way by species subsequently described, but that still 

 the differences between Gammarus, Niphargus, and Crangonyx are much greater than those between 

 many other genera of the Amphipoda. The main interest in the discussion is its bearing on the 

 question — Have all the species classed under, say, Niphargus, arisen from the parent genus, whatever it 

 may be, by one common origin, or have some of them arisen independently in different localities in 

 which they have been isolated ? 



Franz Leydig [73], 1878, in his remarks on the anatomy &c. of Amphipoda and Isopoda makes a 

 good many references to the subterranean forms. With regard to the plumose bristles called " capsules 

 sensitives " by Humbert, he says that they may be sensitive, but they are not capsules, they are modified 

 pores. " In view of the very varying statements of authors on the eye of Gammarus puteanus, he made 

 investigations from which he determines that the optic ganglion is present, but not the eye, though 

 pigment-spots mimicking the eye have led some observers to believe that an eye existed in fact" (quoted 

 from Stebbing [108, p. 481]). In connection with this point I may add that I have occasionally 

 observed similar pigment-spots in the New-Zealand species, in none of which can I find any external 

 trace of eyes except in Crangonyx compactus, which has two or three imperfect lenses ; some of the cave- 

 inhabiting species of Crangonyx from North America also have imperfect external eyes. It seems 

 probable that there may be great diff^erences in the amount of degeneration of the eyes in different 

 species, and perhaps also in individuals of the same species from different localities. 



In remarking on the distribution of and distinctions between Gammarus pulex, de Geer, Gammarus 

 fluviatUis, Rosel, and Gammarus (Niphargus) jmteanus, Koch, Leydig adopts the view of de Rougemont, 

 that Gammarus pulex minufus, Gervais, is identical with Koch's G. puteanus, which may, of course, be 

 correct enough, while de Rougemont's other observations are unreliable. 



F. A. FoREL [39], in 1878, in an account of the fauna of the Swiss Lakes, mentions Gammarus 

 puteanus, var. Forelii, Humbert, as being found in " die tiefe Region " in the Lake of Geneva. 



GusTAV Joseph [66], in 1879, discovered Niphargus puteanus at Venice, and states that their intro- 

 duction into these carefully-covered wells is best explained by the transport of water from the mainland 

 to replenish these wells in the dry season. 



S. Fries [43], in 1879, discusses the occurrence of Well Shrimps in the slightly brackish wells of 



Heligoland, &c., and advocates the view that they must have existed in these localities before the islands 



were separated from the mainland. He examined specimens from these wells of Heligoland, from the 



Falkenstein Caves, from tiie springs running out of the caverns, from the Hilgerhausen Caves, and from 



the depths of the Lake of Geneva, and finds no greater differences than would justify the naming of 



varieties. He therefore follows de Rougemont in uniting them all under the name Gammarus puteanus. 



From this it appears probable that at any rate some of the described species of Niphargus are to be 



looked upon merely as local varieties of one and the same species; but this in no way justifies de Rouge- 



mont's inclusion of Crangonyx subterraneus, Spenee Bate, with the others, and does not support his 



statement as to the elaborate series of metamorphoses passed through by the specimens examined by him. 



In the same paper Fries also discusses the blind Isopod, Asellus cavaticus, Schiodte. He had 



uniformly found this in company with Gammarus puteanus, which, according to de Rougemont, is its 



mortal foe. He agrees with de Rougemont that Asellus cavaticus is related to A. aquaticus, very much 



as Gammarus puteanus is to G. pulex. Judging, however, from the scanty descriptions of Asellus 



cavaticus that I have been able to consult, the connection between the first two appears much closer than 



that between the last two. 



Fries's remarks lead de Rougemont to repeat that he is still of the same opinion [90]. 



Professor Giebel [50], in 1879, mentions the finding of Niphargus puteanus in Halle-a.-S. 



Oscar Grimm [51], in 1880, described a new species oi Niphargus — N. caspius — found in the Caspian 



