Igi2.| J. STEPHENSON : Indian Aquatic Oligochaeta. 223 
take place at a different level on the two sides of the same 
animal. 
As to the number of ventral setae per bundle, this is usually 
three, and I have never seen more except in the one instance shown 
in pl. xt, fig. 4, where one of the bundleshas four. Posteriorly there 
may be only two ; andinsome of the anterior segments also there 
may only be two. Very commonly in the anterior segments there 
were two fully formed setae, along with a half-formed seta, of 
which the proximal end was wanting,—as if it were still in process 
of formation, or perhaps rather as if its formation had been 
permanently arrested at this stage (cf. pl. x1, fig. 4). 
Prebranchial region.—The region between the first gill and 
the mouth is in this species peculiar in several respects, and 
seems to merit special description. It may be recalled that in 
Bourne’s worm dorsal and ventral setae begin at the same level 
(segment ii), the interval between mouth and first setae being 
equal, according to his figure, to about a single body segment ; 
cephalization is therefore limited to the first segment. In the 
allied worm previously described by me there is a considerable 
interval between mouth and first gills; in this interval are situated 
the four first ventral setal bundles (exceptionally only three) but 
no dorsal setae; the gills and dorsal setae, therefore, begin on 
the sixth segment, and the first five segments are * cephalized.’ 
In the specimens now under discussion, there is a moderate 
interval between the mouth and the level of the first gill, 
equal on the average (cf. pl. xi, figs. I, 2 and 3) to the diameter of 
the body at the latter situation. No distinct external annulation 
could be made out in this interval; and the pigmentation was 
not segmentally arranged. The most curious point however is 
the varying distribution of ventral setae in this region. 
The setae are always, when present, of a type distinct 
from those in the rest of the body. In their general proportions 
they resemble the ‘ anterior’ setae, but are considerably shorter 
(77 to 87), remarkably slender, with delicate prongs, and a 
nodulus proximal to the middle of the shaft (proximal : distal : : 
36; stor ease) 
Their distribution was as follows. In one specimen (pl. xi, fig. 1) 
there were four bundles of such setae in the prebranchial region ; 
in another three (pl. xi, fig. 2); the condition in these specimens 
(except for the small size of the prebranchial setae) was therefore 
the same as in the related species previously described by me. In 
a third specimen there were on one side two setal bundles. of 
only one seta each, the anterior of the two being the longer; on 
the other side there was, near the mouth, a single bundle of two 
setae ; the prostomium in this specimen was fairly well formed, 
and the prebranchial region of considerable extent ; i.e. consider- 
ing the manner in which asexual division takes place in this form 
(v. inf.), this specimen had probably been leading a free existence 
for some time. In a fourth there was on one side a single bundle, 
of one seta only, situated nearer to the first gill than to the 
