1912. | J. STEPHENSON : Indian Aquatic Oligochaeta. 227 
and a considerable portion of the process of differentiation of the 
head is completed after separation. In this differentiation, 
the formation of the setal bundles sometimes lags behind that 
of the other structures, and the number of setal bundles formed 
varies considerably. Not infrequently, it would appear, none are 
formed; and the maximum number of four is perhaps compara- 
tively seldom produced. 
(3) Systematic Position. 
Though closely related to the worm which I have described as 
Lahorta hortensis, the present species is not identical with it ; 
and the more restricted distribution of the gills in the form under 
discussion, the details of asexual reproduction, certain differences 
in the form of the setae, and the smaller number of these in both 
dorsal and ventral bundles, are sufficient to distinguish it. 
With regard to Bourne’s Chaetobranchus semperi the agreement 
is in many respects closer. Thus Bourne gives identically the same 
number of segments (130) which I counted in the best developed 
specimens of the present batch ; the details of pigmentation corres- 
pond in the two ; the number and distribution of the gills is about 
the same ; the details of asexual reproduction are strikingly simi- 
lar, and different from what is usual in other Naididae; and 
finally both were taken in the same locality. 
On the other hand there are several points of difference. To 
begin with the less important, the length of Bourne’s worm 
appears to have been greater, though some of the apparent 
difference is no doubt due to contraction of the preserved 
specimens ; the setae did not begin to project freely so soon in 
Bourne’s specimens (about the 30th segment), as in mine (13th to 
28th); and, to judge from Bourne’s figure, the position of the 
mouth is different in the two, the prostomium being considerably 
longer, and the prebranchial region somewhat shorter in Bourne’s 
specimens than in those now under discussion. Further and more 
important differences are found in the characters of the setae; 
though those of the two forms have a general resemblance, 
this does not extend to details (compare, for example, the dorsal 
needles of the present form with the sickle-shaped dorsal setae 
of Chaetobranchus sempert) ; and a marked distinction is found 
in the numbers of setae per bundle, both in the dorsal and ventral 
series. Lastly there is the fact that in the present form there may 
be as many as four pairs of ventral setal bundles between the 
first gills and the mouth. 
In my present specimens, it is only in a minority that well- 
formed setal bundles are developed between the first gills and the 
mouth; and had I received a smaller number of the worms, 
say two or three only, it is not improbable that such examples 
would have been wanting altogether; in which case, in view 
of the many and detailed points of similarity, it is not unlikely 
that I should have recorded the present find as a rediscovery 
