215 
of Edinburgh, Session 1880-81. 
a fine wall, it is possible sucb represents the vessel (or at least 
a division or septum) with its contents. Proceeding distally, both 
sets of vessels (in the double rows) become complete, and the 
hypoderm covers both the outer and inner edges of each — the 
somewhat spongy vascular tissue having disappeared. Then each 
row becomes more individualized — having a thin layer of hypoderm 
on the inner or concave edge, and a prominent pennate mass (in 
section), several times thicker, on the outer. The rings are bound 
together only by a little connective tissue in the middle, and they 
soon become free, — the outer layer of hypoderm still remaining 
thicker, while that on the sides (where the densest portion of the 
chitinous ring exists) is less developed. When viewed longi- 
tudinally, this peculiar chitinous wall has a series of very bold 
and rather regular transverse folds or wrinkles ; and an included 
vessel and its contents are apparent toward the tip. 
The mouth opens at the bottom of the ventral (and outer) whorl 
of the branchial apparatus (which, as formerly mentioned, presents a 
continuous fold in this region) — the arrangement resembling a 
spacious funnel. The first part of the alimentary canal has its wall 
folded transversly in zig-zags, the next and longest region is mostly 
devoid of them, but they again become very well marked in front 
of the dilatation generally found at the posterior end of the body. 
The canal then turns forward along the dorsal aspect to terminate 
in the anus, on the ridge between the two dorsal branchial fans. 
The canal is fixed at its commencement by various fibres which pass 
from the lower curve to the ventral wall of the body, and by a 
strong band (with an elastic central portion) at each side to the 
lateral region of the body. The structure of the glandular lining of 
the canal agrees with that in the Annelids and their allies. Pro- 
ceeding backward the oesophagus becomes fixed by the radiating mus- 
cular fibres passing from the body-wall between the pennate muscles 
throughout the greater part of the circumference. Dorsally, however, 
two strong oblique bands cut off a limited area for the rectum, and 
sling the former region by r their attachment to its dorsal wall. 
These radiating bands would seem to perform an important function 
both in regard to the calibre of the alimentary canal, the vessels, and 
the body-wall. The canal retains similar connections throughout 
the anterior region, though it is more central in position as the body 
VOL. XL 2 E 
