838 FRESH-WATER BIOLOGY 



The fresh-water Amphipods are remarkable for the development of 

 eyeless cave forms; in fact, there is a strong tendency among them 

 toward underground life. Of the 20 species known, 10 or i: seem 

 to be inhabitants of caves, wells, or springs. Not all of them have 

 the eyes reduced, but the species of the genera,Crangonyx,Stygonectes, 

 and Apocrangonyx are actually blind, and there is a bHnd species 

 in each of the genera Eucrangonyx and Gammarus, while the other 

 species of these two genera show all transitional stages from well- 

 developed eyes to more or less reduced eyes. The correlation be- 

 tween subterranean life and reduction of the eyes is very evident in 

 this group. 



The only species of the decapod-famUy At3ddae found in the 

 United States, Palaemonias ganieri (Fig. 13 11), is a bHnd cave-form, 

 and it was discovered only recently (1901) in the waters of Mam- 

 moth Cave in Kentucky. This form has eye-stalks, but the visual 

 elements of the eye are gone. This is an extremely interesting 

 form on account of its primitive structure as well as its geographical 

 relations. Most of the members of this family, which is strictly a 

 fresh-water group, are found in the tropical and subtropical regions 

 of both hemispheres, but a form very closely allied to the American 

 is known from caves in Carniola, Austria. 



In the family Palaemonidae is included Palaemonetes antrorum, 

 which was discovered in an artesian well in Texas. Also this species 

 is provided with eye-stalks, but the eyes themselves are obliterated. 



Within the genus Cambarus of the family Potamobiidae, five 

 cave species are known. They are all bhnd, but the eye-stalks re- 

 main. These species belong to different subgenera, and the best 

 known is the famous blind crayfish of Mammoth Cave in Ken- 

 tucky (Cambarus pellucidus), which is also found in other caves 

 in Kentucky and in Indiana. It belongs to the subgenus Fax- 

 onius, and represents a rather ancient type, so that we are jus- 

 tified in regarding it as an old immigrant into the subterranean 

 waters. Three species (C. kamulatus, C. setosus, and C. ayersi) 

 belong to the subgenus Bartonius, representing a primitive section 

 of it. The first of these is found in Nickajack Cave in eastern 

 Tennessee, while the two others are from caves in the Ozark region 

 in Missouri. These three species also must be old immigrants into 



