some Freshwater Amphipods. 239 



flecks, like tliose I have referred to in connexion witli the 

 specimens from Modena. 



The large pigment-flecks of Crangonj/x suhterraneus, which 

 I have described as a pigment-voil [" Pigmentschleier "], 

 suggest by their position the former presence of eyes; I 

 have not, however, referred to them as rudiments of eyes so 

 long as there was no definite proof that the pigment was 

 directly connected on the one hand witii the crystalline cones 

 and on the other with the optic ganglia. 



We know now, however, that on the one side there are 

 species of Crangonyx W\X\\ normal eyes, and on the other that 

 the eyes of C. compactus from New Zealand are, according 

 to Chilton, only represented by two or three little " lenses " 

 without pigmeTit. If we tarn to C. suhterraneus, we must 

 regard the pigment-veil as a rudiment of an eye in which the 

 crystalline cones have completely disappeared and only the 

 pigment-ccUs remain. We hud, therefore, that the genus 

 Crangonyx is characterized by visual organs in all possible 

 stages of reduction ; and we might expect to And similar 

 series of degenerating eyes in other genera. For Gammarus 

 I have already mentioned the observations of Moniez and 

 Schneider ; a completely eyeless species G. fragilis, has 

 been described from New Zealand by Cliilton, and I myself 

 know a large species from Herzegovina the two examples of 

 which in my possession lack all trace of eyes. 



The same series may be made out in Niphargus. It is 

 true that species with normal eyes are not known, but 

 N. elegans, which is characterized by possessing only the 

 eye-pigment, permits of the assumption that there are species 

 with eyes, and there exists a whole series of completely bhnd 

 species, as N. KocJiianus, Cdspary, puteanus, &c. (Tlie work 

 of Vire has not been accessible to me.) 



Now it is possible that there are eyeless forms which occur 

 at the same time and in the same place with those possessing 

 reduced and normal eyes. At least " Gammarus pulex,^^ 

 from Wildenschwyl in Switzerland, deserves a renewed 

 investigation, as, according to the statement of Asper, it is 

 represented at a depth of 40 m. by individuals with and 

 without eyes. This case of variable development of the 

 visual organs in different individuals of the same species is 

 confirmed among other species of animals by Packard, and 

 I'orel particularly notes that in rare and exceptional instances 

 the blind Asellus Forelii, lilanc, still bears vestiges of eyes ; 

 and in lliis connexion it may be reniembereil that years ago 

 1 fountl in a well in Prague both eyed and eyeless forms of 

 the rhabdocccl Gyrutor uotops, Dugbs. The eyeless form 



