38 THE SHORTER SCIENTIFIC PAPERS 



The genus Nais furnishes one of the most diflficult 

 problems for the systematist attempting to define the 

 limits of species among the Naidid^e. The following 

 table, however, embodies the results of systematic work 

 so far as they are known and comprises all species described 

 up to the present time. 



A^. Set£e of ventral bundle of segments 8 to 10 neither thicker than 

 those of other segments nor modified by possessing blunt tips 

 with rudimentary lower tooth. 



B-^. Eyes present. 



C^. Ventral set^e of segments 2 to 5 much longer than those of 

 succeeding segments. Dorsal setae capilliform, 4 to 8 in 



bundle N. obtusa. 



(Europe, S. Siberia.) 

 C^. Ventral setx of segments 2 to 5 not decidedly longer than 

 those of succeeding segments. 

 D^. Transverse blood vessels simple. 



E^. Ventral setse of segments 2 to 5 unequally bifid at tip. 



Length 1 to 1.5 mm N. elinguis. 



(Europe, N. America, S. America.) 



E^. Ventral setae of segments 2 to 5 equally bifid at tip. 

 F^. Number of segments in an individual approximately 

 10 (9 to 10), 6 to 8 ventral setae in a bundle. 



Length of specimen 1.5 mm N. parvula. 



(N. America.) 

 F^. Number of segments in an individual usually 20 (18 

 to 22), 4 ventral setae in a bundle. Length of 

 specimens at least 2 mm. 

 G^. Dorsal bundles composed of 1 long capilliform 

 and 2 short needle-like sets. Eyes dumbbell- 

 like in form N. tortuosa. 



(N. America.) 

 G^. Dorsal bundle composed of 1 long capilliform and 

 1 short biuncinate seta. Eyes oval, not dumb- 

 bell-like in form. Length 3.5 mm. N. parviseta. 



(N. America.) 



D^. Transverse blood vessels of segments 2 to 5 forked. 



Dorsal bundle with biuncinate setae. . . . N. heterochaeta. 



(England.) 

 B^. Eyes absent. 



C^. Proximal tooth of dorsal biuncinate setse not longer than 

 the distal tooth. 

 D^. Ventral bundle composed of 3 to 4 setse. Length of 

 individuals 3 to 3.5 mm. Colorless . . . . N. tenuidentis. 



