some Freshwater AmpMpods. 239 



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

 specimens from Modena. 



Tiie large pigment-flecks of Crangonyx suhterraneus, Avhicli 

 I have described as a pigment-veil ['' Pigmeutschleier "], 

 suggest by their position the former presence of eyes; L 

 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 with 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 with normal eyes, and on the other that 

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

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

 without pigment. If we turn 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-cells remain. We find, therefore, that the genus 

 Crangonyx is characterized by visual organs in all possible 

 stages of reduction ; and we might expect to find 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 Chilton, and 1 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 blind 

 species, as N. Kochianus, Caspary, puteanus^ &c. (The 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 Wiidenschwyl 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 

 Forel particularly notes that in rare and exceptional instances 

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

 and in this connexion it may be remembered that years ago 

 J found in a well in Prague both eyed and eyeless forms of 

 the rhabdocoel Gyrator notops^ Dug^s. The eyeleds form 



