214 
Proceedings of the Royal Society 
one being on the outer edge of the volution and tbe other on the 
inner — a chitinous basement-layer separating the former row and its 
spongy vascular tissue from the latter. A line intersecting the 
coils would thus (before separation occurs) pass through four 
complete series of these channels, and in certain positions (involv- 
ing the central volution) through six. At the base the inner row 
at the tip of the central coil is incomplete, but by and by 
the vessels form a continuous series round the edge. The structure 
of one of these double basal rows is as follows : — Externally is the 
cuticular layer, which, however, in the preparations does not seem 
to be well differentiated, since it forms only a definite boundary to 
the hypoderm. This feature is probably related to the branchial 
functions of the region. Within is a layer of hypoderm, which 
varies in thickness and colour according to the level of the section. 
Its structure agrees with the hypoderm of the body- wall, and accords 
with the same tissue in the Annelids and bTemerteans. Directly 
under it are the chitinous arches of the outer row of channels, 
which are incomplete internally, eaeh debouching into a large 
vascular channel, which with connective tissue fills up the space 
between the outer row and the line of basement-tissue separating 
the two series. In transverse section the chitinous tissue shows 
the thickened sides formerly alluded to, and is often finely streaked, 
but the latter is probably only an optical peculiarity. Within the 
line of basement-tissue are the inner series of channels, which 
have complete walls (and the outer row would thus seem to be 
less developed at a given level inferiorly than the inner) ; and in 
transverse section present a noteworthy uniformity in appearance, 
viz., with a dumb-bell-shaped outline of the inner wall from the 
thickening of the median region or each side, while the lines of 
the outer wall laterally are nearly parellel. The outer and inner 
arches of the vessel are thinner. Only a slight quantity of 
connective tissue separates each chitinous channel from the septum 
(line of basement-tissue), while the other arch has a thick coat of 
hypoderm, which soon (as we proceed distally) becomes grouped in 
a series of fan-shaped masses. The inner surface of the chitinous 
channels is furnished with an epithelial lining ; and since in section 
the arch next the septum generally has a rounded opaque mass of the 
circulating fluid, surmounted in many by a granular band as if from 
