456 PHILOSOPHY OF ZOOLOGY. 



responding with the centre of the shield in the slugs, there 

 is (as CUVIER has characteristically termed it) a natural 

 rupture, through which the viscera are protruded into a co- 

 nical bag twisted spirally. In this bag are contained the 

 principal viscera, the liver occupying its extremity. The 

 body of the animal is attached to the pillar of the shell by 

 a complicated muscle, which shifts its place with the growth 

 of the animal. The mouth is furnished above with a thin- 

 arched corneous mandible, notched on the edges. The 

 whole body, including the foot and head, are, in general, 

 capable of being withdrawn into the cavity of the shell. In 

 one genus, the aperture is closed by a lid. 

 1st Tribe. 



The foot is furnished with a lid or operculum, for closing 

 the mouth of the shell when the animal withdraws itself in- 

 to the cavity. 



20. CYCLOSTOMA. Aperture of the shell circular. 

 The tentacula are linear and subretractile. The primary 

 ones have subglobular, highly-polished extremities, consi- 

 dered by MONTAGU as the eyes. The true eyes, how- 

 ever, are placed at the exterior base of the large tentacula, 

 and are elevated on tubercles, which are the rudiments of 

 the second pair. 



The aperture of the pulmonary cavity is situate on the 

 neck. The sexes are likewise separate; the penis of the 

 male being large, flat, and muscular. The mouth is form- 

 ed into a kind of proboscis, and the upper lip is deeply 

 emarginate. The Turbo elegans of British conchologists 

 is the type of the genus. 



2d Tribe. 



Foot destitute of a lid. 



A. Aperture of the shell with a thickened margin. 

 In all this division, the margin of the shell, while the ani- 

 mal is young, is thin ; but, upon reaching a certain period, 



