ON MOLLUSCA. , Set 
symmetrical or not symmetrical, act in somewhat a different 
way. In fact, in the first case all the veins of the body unite 
into a single gross trunk, which most usually, without the 
intervention of a muscular swelling or heart, is subsequently 
changed into a pulmonary or branchial artery. In the second 
case, on the contrary, the veins unite into two principal trunks, 
which are subdivided into as many branchial arteries as there 
are gills. At the point of this transformation there is never 
any true heart or organ of impulsion, but in all the branchio- 
cephala, and even in a small number of cephalous species, 
and probably in the acephala, we find in this place a venous 
sinus, to which the name of heart has been sometimes given, 
but which probably cannot be so designated, as it possesses 
no muscularity. Be this as it may, the branchial or pulmo- 
nary artery, simple or multiple, is ramified in a manner more 
or less regular, according to the form of the respiratory organ 
in the modified skin which constitutes it. 
It is from the capillary extremities of the branchial artery, 
subdivided in the respiratory organ, that the second venous 
system originates. After that the ramifications, disposed like 
those of the arteries, have been successively united in 
branches more and more thick, there results from them finally 
a gross trunk, which issues from the respiratory organ, and 
repairs into an aortic heart situated in a different manner, ac- 
cording to the position and symmetry of the gills. 
The heart of the mollusca, in the greater number of cases 
situated in the back, above the intestinal canal, unless per- 
haps in the brachiocephala, where it is inferior, is placed, as 
in osseous animals, at an equal distance from each respiratory 
organ when there are two, or obliquely to the left, and rarely 
to the right, when the organ is odd. ‘This heart is not con- 
tained in a true pericardium, but in a muscular lodge of the 
diaphragm kind, which separates the visceral cavity from that 
of the gills. As to the rest, it is formed of an auricle, some- 
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