HIGHER CRUSTACEANS (MALACOSTRACA) 



841 



KEY TO NORTH AMERICAN FRESH-WATER MALACOSTRACA 



1 (26) Without carapace, but first thoracic somite coalesced with the head. 



Eyes (when present) sessile. Thoracic limbs without exopo- 

 dites, first pair modified as maxillipeds 2 



2 (n) Body depressed. Pleopods biramous, uniform in shape, with excep- 



tion of the uropods and the anterior pairs of the male. 



Order Isopoda . . 3 



3 (4) Uropods lateral, forming with the telson a tail-fan. 



Family CIROLANIDAE. 

 Only one genus and one species in the United States. 



Cirolanides texensis Benedict 1896. 



This is a blind form, which has been found in an arte- 

 sian well in Texas. All other representatives of this 

 family are marine. Many of them are ectoparasites on 

 fishes. 



FIG. 1304. Cirolanides texensis Benedict. X 4. 

 (After Richardson.) 



4 (3) Uropods inserted at the posterior end of the telson, not forming a tail- 



fan 5 



5 (10) Pleopods covered by a thin opercular plate, the modified first pair. 



Body symmetrical. Free living. Family ASELLIDAE . . 6 

 This is a typical fresh-water family. 



6 (7) Mandibles without a palp. Last six pairs of thoracic legs with dacty- 



lus biunguiculate Mancasellus Harger. 



Five species, living in springs and caves, some in rivers and lakes. Eyes present in all, but 

 small. 



7 (6) Mandibles with a three- join ted palp. Last six pairs of thoracic legs 



with dactylus uniunguiculate 8 



8 (9) Eyes present. Head narrower than the first thoracic segment. Telson 



not longer than broad Asellus Geoffrey. 



Seven species in rivers, creeks, ponds, ditches, 

 springs, lakes. Some (as Asellus communis Say) 

 widely distributed, others more local. Common in 

 ponds, ditches, etc., living among decaying vegeta- 

 ble matter. 



FIG. 1305. Asellus communis Say. X 2. (After Smith.) 



