246 CHITONID 2. 
scending aortic vei of similar calibre, as much blood would 
be sent to avery small area as to one seven times more 
extensive, and so great an inconvenience would have resulted, 
that nature has created this peculiar mode to effect a just 
distribution. I hope I have almost proved that this anomalous 
structure is a consequence of the posterior position of the 
heart. 
I have extended these remarks somewhat beyond what is 
necessary, but I am anxious to show that this curious con- 
trivance to effect a particular object has nothing im it essen- 
tially contrary to the molluscan type; in other respects, as in 
it, the circulation is aortic, venous, and particular, the blood 
being brought from the body by the venz cave to the great 
arterial vein of the respiratory organ, from whence it is distri- 
buted to its minor arteries, and after aération reverts by the 
branchial vein to the auricles and heart, to repeat, as long as 
life lasts, the same course; consequently it is completely 
molluscan, and appears more advanced in composition than 
that of the Annelida, by the presence of a much more effective 
motive power of the heart and auricles, which in the Articu- 
lata are comparatively obsolete, or mere continuous fluctuating 
cylinders or inflations. A short cesophagus conducts to the 
stomach, which is an irregular subcylindrical cavity, about 
double the diameter of the pyloric extremity ; it traverses the 
body, forming a sudden curvature like the doubling of a horse- 
shoe, and returns across the body with the posterior portion 
parallel to the anterior one, commencing at the pyloric orifice 
a very long intestine of five or six transverse or oblique folds, 
supported by the liver, and disemboguing as the rectum, at 
the centre of the posterior extremity between the bran- 
chie. This is quite different in the Articulata, m which the 
intestine runs without inflexions through the middle of the 
body. 
For further particulars we refer to the description of the 
type, as well as for the liver, ovary, and foot, all of which pre- 
sent no essential variation from the molluscan type, except the 
double oviduct, if such be the case. 
The Chitons are best illustrated by the Patelloid section 
