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LAMELLIBRANCHIATE CONCHIFERA. 91 
auricle into the ventricle ; valves also exist at the origin of the aorte. It is well known 
that the rectum commonly pierces the ventricle, and that in the Oyster there is an 
exception to the general rule. In the Anomia the ventricle lies upon the rectum’. In 
Arca, Lima, and some Pectunculi, there are two ventricles, the rectum passing between 
them. There is nothing remarkable in the distribution of the arteries. Sometimes, as 
in Cardium echinatum, the first part of the aorte appears very wide, having strong 
parietes with numerous fleshy columns on their internal surface. 
The veins then do not all go to form the branchial arteries; whilst some of the 
venous blood enters directly the auricles, another portion is first distributed to the 
secreting organs. There also exists a reservoir, or sinus, or set of very large veins 
receiving much of the venous blood, and which appears to empty itself both into the 
veins of the branchie, and into those of the secreting organs. The venous blood from 
the body in some measure, likewise, enters the auricles. The branchial arteries receive 
few veins, but the principal supply to them is from the sinus. Some resemblance to a 
portal circulation is seen in this, though it has not place in the liver, in which the bile 
is secreted from the arteries. 
Blood taken from the auricles is almost colourless, separates on standing into a 
liquid and solid part, and, microscopically examined, presents a curious phenomenon, 
which appears to have been observed by Mayer ; viz. its globules, which are about the 
one thousandth part of an inch in diameter, appear with projections upon them, showing 
an evident motion. The heart is slow in its pulsations ; they are generally from twenty 
to thirty in the minute. 
Respiratory System. 
I am disposed to regard the disposition and form of the branchi@ and siphons as being 
of great use in the classification of these animals ; for instance, Anomia, Pecten, Arca, 
Modiola, Unio, Cardium, Cyclas, Donaz, and Mactra, have each a particular disposition 
of the branchie, sac, of the mantle, valves, siphons, &c. giving rise to particular mo- 
difications of the course of the aérating currents of water to the branchie. In all 
the above genera, no complete division of the sac of the mantle exists. However, in 
Solen, Hiatella, Pholas, &c. a different disposition takes place ; here the branchie are 
prolonged into the inferior siphon, and as they are not separated from the base of the 
foot within, nor from the mantle without, the water drawn in through the inferior ori- 
fice must make its exit by the same, or by the anterior opening. But water is likewise 
drawn in by the superior siphon, and so gets access to the interior interlaminary spaces 
of the branchie (oviducts of some) ; and by this superior siphon, the ova, feces, and 
secretions are discharged*. Here the branchie are often very long, and the siphons 
1 I do not find that the rectum in the Mya Pictorum perforates the ventricle to make its exit and re-enter 
again, as stated by Dr. Grant; Lect. Anat. Comp. Lancet, vol. ii. (1833-4), p. 708. 
2 We see this also in the Tunicata. By one orifice water enters the respiratory sac, by the other it is drawn 
