10 MOLLUSCA. 



they are true branchiae, 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 branchial cavity is always more or less oval, 

 but in the aquatic species it is found simple or compound. It 

 consists of numerous ramifications in the Tritonice, of tufts or 

 laminae in other genera, of triangular pyramids in the Loligines, 

 &c. In many genera it is external, as in the Pteropoda, 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 respiratory organ has in 

 most species a considerable resemblance to that of fishes, con- 

 sisting 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 short, with almost always two jaws. In others it consists 

 of a retractile proboscis, furnished with small teeth at its inter- 

 nal orifice, and has no jaws. Those which have jaws have 

 the mouth sometimes vertical, presenting two horny edentate 

 jaws hooked like a parrofs bill, sometimes placed under the 

 head or almost at its anterior extremity, and 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 tentacula 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 appear- 

 ance of a proboscis. In this latter case, however, it is al- 

 ways distinct from the true proboscis, which has no jaws^ and 

 is retractile. The two jaws of the kind of snout just mention- 

 ed 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, 

 but much smaller in others. It is fleshy, muscular, contractile, 

 and pliant. Its extremity is perforated by a round hole, mar- 

 gined by cartilaginous membrane, and armed with very small 

 teeth. 



The Proboscidiferous Mollusca are carnivorous, making use 

 of the organ for perforating the shells of other animals and 



