MOLLUSCA. 755 



edly 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. Inmost of them they are true bronchiae, 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 aquatic 

 species, it is found simple or compound. It consists of numerous ramifica- 

 tions in the Tritonia, of tufts or laminae in other genera, of triangular pyra- 

 mids in the LoUgines, &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 arid 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 laminae, 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 tentaculae 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 maxillae, there are some which have, instead of them, 

 a kind of cylindrical tube, of great length, in certain species, hut 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 



