484 PHILOSOPHY OF ZOOLOGY. 



cealed by the animal, by means of the reflected margins of 

 the foot, and its appendage. Along the right side of the 

 body there is a groove, formed by the foot and its append- 

 age, on one side^ and the dorsal plate and tentacular disc on 

 the other. The branchiae are situate in a cavity under the 

 shell or dorsal plate, and resemble those of the Aplysia. 

 Behind the gills, in the lateral groove, is the anus ; and in 

 front of these, the orifice of the united organs of generation. 

 The penis is removed as in the Aplysia, and connected by 

 a similar slit. 



The mouth is, as usual, in front, above the foot and be- 

 neath the tentacular disc, both of which serve as hps. The 

 cheeks are strengthened on each side by a corneous plate. 

 The tongue is well developed in some, as the B. ampulla^ 

 while in the B. apcrta it is reduced to a small tubercle. 

 The gullet is large, and in the B. lignaria makes two folds 

 before entering the gizzard. This last organ is fortified by 

 three testaceous plates, convex and rough on the inner sur- 

 face, and attached to strong, muscular walls. These plates 

 exhibit in the different species considerable varieties of form 

 and markings. The intestine, before terminating in the 

 anus, makes several convolutions in the substance of the 

 liver. The salivary glands exhibit considerable differences. 

 In the B. ampulla, they are long and narrow, and their 

 inferior extremity fixed to the gizzard. In the B. aperta 

 and lignaria, they are short, with the extremity free. In 

 the B. hydatis they are long, unequal, and the extremity 

 of the one, belonging to the left side, is forked. The liver 

 forms a part of the contents inclosed in the spire of the shell. 

 It envelopes the intestine, and empties the bile into its pylo- 

 ric extremity. 



The auricle and ventricle appear to occupy the same re- 

 lative position as in the Jplijsia, but the structure of the 

 arteries is unknown. The organs of generation have also 



