NOTE ON A NEW SPECIES OF THE GENUS NATS. 383 



Note on a New Species of the Genus Nais. 



By 



!¥. Blaxland Benliam, D.Sc.LiOiid., 



Aldrichian Demonstrator of Comparative Anatomy in the University of 

 Oxford. 



With Plate XXXIII. 



In a gathering from a ditch in the neighbourhood of 

 Oxford I found several specimens of a species of Nais, which 

 present features diflfering from those recorded for any species 

 of which I have been able to find a recognisable description. 



Nais heterochaeta, n. sp., is a small worm about a quarter 

 of an inch in length, of a dull brownish colour. I have not 

 yet come across any sexually mature forms, or specimens 

 exhibiting zones of budding; the number of somites varies 

 from about thirty-one to about forty-one in specimens which 

 I have observed from this point of view. 



The dorsal bundles of chsetse have a characteristic composi- 

 tion, which the specific name is intended to indicate. Nor- 

 mally there are only two chgetse to a bundle — cases with 

 three being very rare. These two chaetse are of diff'erent shape 

 and size, (a) One is capilliform, but comparatively short, 

 being -165 mm. in length, not as long as the body is broad ; {b) is 

 a small furcate or " crochet," measuring "045 mm. (fig. 2). 



The length of the capilliform chsetse varies slightly in dif- 

 ferent regions of the body; those of the Somites vi and vii 

 being shorter than the above length, and those of Somite xii 

 exceeding it to a slight extent. They gradually increase in 



