MOLLUSCA. 755 
ed.y uniting into trunks, arrive at the respiratory organ, where they are con- 
verted into an arterial system, which ramifies through its substance. From 
the capillary extremities of the bronchial artery, arises the second venous 
system. The veins unite into branches, which terminate in a large trunk, 
pouring its contents into the heart. The color of the: blood is white or 
bluish 
The organs of respiration vary considerably, not only in respect to their 
form, and the place which they occupy in the animal, but also with respect to 
structure. In most of them they are true bronchie, or receive the influence of 
the ambient fluid on their surface; while in some others, they form a sort of 
cavity, into which it penetrates, as in the terrestrial mollusca. In the latter 
animals, the bronchial cavity is always more or less oval; but in the aquatle 
species, it is found simple or compound. [ft consists of numerous ramifica 
tions in the Tritonie, of tufts or lamine in other genera, of triangular pyra- 
mids in the Loligines, &c. In many genera it is external, as in the Ptero- 
poda ; while in others it is more or less internal. It is sometimes situate at 
the upper and posterior part of the body, as in the genus Doris; at other times, 
on each side of the back; most commonly, however, at the anterior and 
superior part of the commencement of the back. The structure of the re- 
spiratory organ has, in most species, a considerable resemblance to that of 
fishes, consisting either of triangular lamina, like the teeth of a comb, or 
of granulations, or tubercles, arranged along a common axis. 
The mouth is in general armed with hard parts. In some it is shut, with 
almost always two jaws; in others it consists of a retractile proboscis, fur- 
nished with small teeth at its internal orifice, and has no jaws. Those 
which have jaws, have the mouth sometimes vertical, presenting two bony, 
toothless jaws, hooked like a parrot’s bill, sometimes placed under the head, 
or almost at its anterior extremity, or very small. It presents itself under 
the form of a longitudinal or transverse fissure, and terminates that part of 
the head which extends from the base of the tentacule to the aperture of 
the mouth, and which is named the snout. This snout is sometimes very 
short, and sometimes so elongated as to assume the appearance of a 
proboscis. In this latter case, however, it is always distinct from the true 
proboscis, which has no jaws, and is retractile. The two jaws of the kind 
of snout just mentioned, are cartilaginous and very unequal. Among those 
_ which are destitute of maxille, there are some which have, instead of them, 
a kind of cylindrical tube, of great length, in certain species, but much 
smaller in others. It is fleshy, muscular, contractile, and pliant. Its ex- 
tremity is perforated by a round hole, margined by cartilaginous membrane, 
and armed with very small teeth. 
The proboscidiferous mollusca are carnivorous, making use of this organ 
for perforating the shells of other animals, and sucking their flesh. Those 
which have the parrot beak, are also carnivorous. Those which have a 
