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. 
Crass I.—PuLMoNIFERA. 
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 deztral, 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- 
