ANATOMY OF THE EARTHWORM. 105 
sural vessels. In order to understand well the visceral system it 
is necessary to consider it as being composed again of two dor- 
sal and two sub-intestinal trunks, each pair having a tendency, 
in the various modifications met with in the ¢lass, to coalesce 
and form a single vessel. The two dorsal vessels, when they 
are developed, are connected by transverse vessels, called 
“superior commissural vessels;” the two sub-intestinal vessels, 
or the one which they form, also communicate with the 
dorsal vessel by lateral branches, called ‘“‘ deep commissural 
vessels ;”’ and, again, the capillaries of the cutaneous system 
communicate with the dorsal vessel, or it with them, by the 
“ Jatero-dorsal” vessels, thus establishing a general communi- 
cation. 
Having thus seen what is the general arrangement in the 
Annelida, let us turn to the earthworm. If killed with chlo- 
roform, the worm presents on the dorsal surface of its 
alimentary canal, when opened, a rhythmically moving dark- 
coloured vessel, generally containing a considerable amount 
of the red fluid. This is the dorsal vessel of the visceral 
system, and extends from the eighth segment throughout the 
body (Part I, P|. VII, fig.5, and Pl. TT, figs. 1, 2,3, a). Beneath | 
the intestine, but not closely attached to it as is the dorsal 
trunk, is the single sub-intestinal vessel (PI. VI, figs. 1, 2, 3, 5), 
also with highly contractile walls, but generally not to be 
observed in movement, inasmuch as the dissection required 
before it is reached destroys the lingering vitality of the 
worm. The sub-intestinal vessel extends throughout the 
whole length of the body excepting the first four or five 
anterior segments, where it is broken up into capillaries. 
The dorsal and sub-intestinal vessels are connected by the 
lateral “deep commissural vessels” (PI1.VI, figs. 1, 2, 3,/), thus 
completing the visceral system of M. Milne Edwards. Of the 
deep commissural vessels there are two in each segment poste- 
rior to the sixteenth, embracing the alimentary canal; those 
from the sixteenth to the nineteenth, in connection with the 
stomach and gizzard, dividing into a wonderfully fine capil- 
lary plexus, the vessels of which in the fibrous gizzard have a 
horizontal parallel disposition. From the nineteenth ring 
backwards the deep commissural vessels consist merely of a 
pair in each segment, closely attached to the wall of the 
intestine, and imbedded in or covered up by the yellow 
granular substance which is spread over its surface and is 
supposed to have a biliary function. Anteriorly to the six- 
* teenth ring the deep commissural vessels are single; in fact, 
the two are fused into one, and, until the seventh segment is 
reached (when they cease to exist or become as the dorsal 
ve 
