MOLLUSCA. 147 



ing the superior one. The pillar is in some cases nearly 

 solid, in other instances tubular, with its base either open 

 or covered. When the base of the tube of the columella is 

 uncovered, the opening is termed the pillar cavity, or um- 

 bilicus. 



In general, when a spiral shell is placed upon its base or 

 mouth, with the apex towards the observer, the mouth will 

 be found situated on the right side, and the whorls will be 

 observed revolving in a direction from right to left, or cor- 

 responding with the motion of the sun. These shells are 

 termed dextraL A few species have this order reversed, as 

 the observer will readily perceive. For upon placing the 

 shell in the above-mentioned position, the mouth will be 

 found situated on the left side, and the whorls will revolve 

 from left to right. These shells are termed sinistral, hete- 

 rostrophe, or heteroclite. 



CLASS I PULMONIFEBA. 



The pulmonary cavity is single and lateral. Its orifice is 

 capable of being closed at the will of the animal. The 

 bloodvessels are spread, chiefly on the walls and roof, like 

 delicate net-work. The opening of the cavity is usually on 

 the right side, with the anus behind it, and the sexual ori- 

 fice is in the front near the head. In some of the genera, 

 these openings are situate on the left side. The shells of 

 the former are denominated dextral, of the latter sinistral. 

 This change in the position of the external openings is ac- 

 companied by a corresponding alteration in the arrangement 

 of the internal organs* The heart, for example, is always 

 placed on the side opposite the pulmonary cavity. In the 

 dextral shells, therefore, it is sinistral. In both kinds, how- 



