318 ALFRED GIBBS BOURNE. 
contact with the dorsal vessel, intestine, ventral vessel, and the 
lateral longitudinal vessels (only in the region where these exist 
of course), the nerve-cord itself runs freely through the septa, 
there being a space left all round through which the cavity of 
one segment communicates with that of the adjacent segments. 
Mesenteries.—I apply this term (it is using the word in 
a forced sense) to a set of mesentery-like structures which 
attach various organs to one another or to the body-wall 
(fig. 41), In the interseptal regions the dorsal vessel and the 
ventral vessel are each attached by a longitudinal mesentery 
to the intestinal wall, so that these three structures run through 
the segment attached to one another, but not attached to any 
other structures. The lateral longitudinal vessels are attached, 
along their whole course through each segment in which they 
occur, to the body-wall in the ventral region by longitudinal 
mesenteries, but are otherwise unattached. The line of 
attachment of these last-mentioned mesenteries begins on the 
hinder surface of the septum near the middle line, at the level 
of the dorsal vessel, and passes along the septum outwards and 
downwards and then inwards, curving round to the inner seta 
line, and then back along the wall of the segment just inside the 
inner seta line, and then up on the anterior face of the hinder 
septum of the segment in which they lie towards the dorsal 
vessel. Prolongations from these mesenteries hold the ne- 
phridia, the hearts, and where necessary the spermathecz 
and their ducts, and the vasa deferentia. 
Behind the region of the lateral longitudinal vessels, and 
therefore of these special mesenteries, there are much smaller 
mesenteries attaching the nephridia to the posterior face of 
each septum. 
All the septa and these mesenteries are lined on both sides 
with irregularly-shaped pavement-epithelium cells. The thin 
septa consist of little else, a few straggling muscular fibres and 
a little connective tissue, but the four specially thickened septa 
and the septa in the posterior region of the body contain 
a much larger amount of muscle; on each surface there is a 
layer of centripetally placed fibres, while the main mass of the 
