40 THE CHICAGO ACADEMY OF SCIENCES. 
vided with an operculum which is situated on the posterior 
dorsal portion of the foot, and.is secreted by a portion of the 
skin known as the opercular mantle. The operculum is horny 
in the fresh-water shells and is used to close the aperture of the 
shell when the animal withdraws. The shell is, with but few 
exceptions (Limax) external and capable of holding the entire 
animal within it. The epidermis in most fresh-water gastro- 
pods is thick and heavy and protects the shell from the erosive 
agency of the carbon dioxide. The effect of such erosion is 
seen in the truncation of the spire of such mollusks as Elimia, 
Pleurocera and Campeloma. Land mollusks are not so much 
affected by this gas. In Limnea and other shells with long 
spires, the parts of the shell appear a trifle different, although 
the names applied are the same. These are sufficiently shown 
in the cut (Fig. 5) and need not be dwelt upon. 
Fic. 5. 
Limna, showing different parts of shell. a, apex; sp, spire; s, suture; 
ap, aperture; p, peristome ; c, columella; lw, last whorl; u, umbilical 
region. 
The external appearance of the animal presents the follow- 
ing characters (Fig. 6): The body is long and narrow and is 
differentiated into a head (h), tail, foot (f) and back, the latter 
covered with glandular tubercles arranged longitudinally. The 
head is connected with the body by a broad neck (n), and sup- 
ports two eye-peduncles (ep), which bear at their extremities 
black, rounded eyes (e). Beneath the eye-peduncles are two 
short, blunt tentacles (t). The mouth is situated on the under 
side of the head in the middle line. The anal aperture is sit- 
uated just above the mouth, a little to the right and a trifle to 
the rear. 
The radula has now become such a factor in the study of 
the Mollusca that a knowledge of its characters is quite essential. 
As this organ is described for nearly all the gastropods cited in 
