316 ALFRED GIBBS BOURNE. 
regularity (figs. 27 and 49). Without careful attention to 
these points one might be very easily led into error. The 
sete are so small that unless well protruded they are often not 
visible in the anterior region, where the body-wall is so thick, 
and the annular grooves are in places so pronounced, that 
from an inspection of the cut edge of the body-wall it is 
impossible to say with certainty which grooves serve as 
demarcations between neighbouring segments and which 
between neighbouring anunuli only. 
It is well known that in some worms certain of the septa 
may be absent, but here all the septa are present, only some 
become displaced; and I need hardly say that in all cases the 
septa should be numbered with regard to their relation to such 
organs as occur in continuous series (e. g. nerve-branches, 
nephridial ducts), rather than with regard to the position of 
their attachment to the body-wall. 
The four thickened septa above mentioned join the body- 
wall in the region of the intersegmental grooves, the most 
anterior, v-v1, showing, however, a tendency to run a little 
forward along the body-wall. 
I can recognise three structures in front of these which 
might be the septa 11-111, 111-Iv, and 1v-v, but these are attached 
(see fig. 17) to Septum v-v1, and never reach the body-wall, 
and are therefore not septa, but special mesenteries supporting 
the nephridia, such as are connected with the other nephridia 
(see below). 
The only septa with regard to which there is any real 
difficulty are 1x-x, X-x1, and XI-xII. 
Septum Ix-x is attached to the body-wall in the dorsal 
region near the intersegmental groove x-x1, while in the 
ventral region it is attached in its normal position. This is 
the septum which supports the seminal capsule, and through 
which the vas deferens passes. At the spot where the excretory 
duct of the nephridium of Segment x penctrates the body-wall, 
this segment is carried forward so as to leave nephridium 10— 
i. e. the nephridium of Segment x—entirely behind it. 
Septum x-x1 is the most modified; the greater portion of it 
