AND ZOOGEOGRAPHY OF INDIAN OLIGOCH.^TA. 129 



septa 10/11 a-ncl 11/12 only a single septum, wliicli is to be 

 regarded as a product of the fusion of the two. More ventrally 

 these separate, so that in a longitudinal section taken to one side 

 of the alimentary canal, segm. xi appears as a wedge-like space 

 with its apex upwards and base at the parietes. This wedge- 

 like space is continuous above the alimentary canal with the 

 similar space on the other side. 



There are, however, other complications in the form of this 

 chamber which are difficult to describe in a few words. If the 

 above were a complete description, it is obvious that segm. xi 

 would be an ordinary segment, except that its hounding septa 

 would be fused dorsally ; and it would contain a section of the 

 alimentary tube, doisal and ventral vessels, and ventral nerve 

 cord. What actually happens may be described by saying that 

 {a) throughout the whole of the gizzard and reproductive regions 

 of the worm the septa are incomplete in their mid-ventral por- 

 tions — around the ventral vessel and ventral nerve cord — as if a 

 series of arches were cut out of them ; (b) that all round the 

 alimentary canal, between septa 10/11 and 11/12, a ring-shaped 

 partition isolates an annular coelomic space (figs, 4 & 6, ann.) 

 round the tube ; this annular cavity contains also the dorsal 

 vessel {d.v.), but the ventral vessel is below it ; (c) longitudinal 

 and almost vertical partitions are also established, extending in 

 the ventro-lateral region between septa 10/11 and 11/12, and thus 

 isolating the mid-ventral portion of the coslomic cavity here 

 (fig. 6, v.ch.). 



The cavity of segm. xi is thus altogether absent dorsally ; and 

 what exists of it is divided up as follows : — (i.) An annular space 

 immediately surrounding the alimentary canal and containing 

 also the dorsal vessel ; (ii.) a larger space, the ovarian chamber 

 (figs. 4 & 6, o.c), surrounding this in turn, of which (iii.) the 

 mid-ventral portion is isolated by longitudinal partitions on 

 each side of the ventral vessel and ventral nerve cord, but is 

 continuous in front and behind with the ccelomic cavities of 

 neighbouring segments *. 



The ovarian chamber contains the ovaries with their funnels, 

 and the nephridia. The ovisac (fig. 4, sac) is an evagination of 

 its posterior wall. The ovaries are proliferations of the epithelium 

 covering the anterior wall of the chamber. The funnels ( /") are 

 constituted by modifications of the epithelium of the posterior 

 wall. 



The walls of the chamber have the usual constitution of 

 septa — peritoneum on the outside, muscular and connective 

 tissue composing their substance. The inside of the chamber is 



* There are other peculiarities of the septa in the neighbouring regions which are 

 diiScult to make out. Thus the floor of the annular chamber round the gut in 

 segm. xi is continued forward and backward as a sheet of tissue whicli extends from 

 behind the gizzard region to the spefmatliecal segment. This sheet maj^ have free 

 Literal margins, or the lateral margins may be bent upwards to join the ventro-lateral 

 wall of the cesophagus. 



Proc. Zool. Soc— 1922, Mo. IX. 9 



