330 SUPPLEMENT 
part of the body, by means of which the animal crawls. All 
the lower surface of the body is smooth, but the upper is 
rugose. The head, especially on the upper part, is not really 
distinguished from the body, except by the organs with 
which it is provided. There are two pairs of the tentacula, or 
horns, the posterior of which are the largest, and have a small 
black point at the extremity which has been considered as an 
eye. They differ much from those of other mollusca, by 
being retractile. At the anterior extremity of the head, is a 
folded aperture which forms the mouth. In this anterior 
part, at the root of the right horn, is a small cleft, where the 
apparatus of generation terminates. The visceral mass is 
entirely concealed by the shell. Of the latter, a description 
would be superfluous. It is attached to the rest of the body 
by the retractor muscles of the head and foot. Some of these 
shells are variegated, and very beautiful, but in general they 
are plain enough. 
The skin of the snail, where it is uncovered by the shell, is 
of an extreme degree of sensibility, and accordingly it re- 
ceives a great number of nerves. It must have a very con- 
siderable number of mucous pores, if we may judge by the 
great quantity of viscous or mucous matter which it ejects. 
This quantity, however, is less than in the slugs. That por- 
tion called the col/ar, which is a muscular ring, bordering 
the visceral mass, and which answers to the mantle of the 
other mollusca, has the greatest number of these pores. 
Thus the general sense of touch, must be, and is, in fact, 
extremely delicate in those animals. We must add, that the 
skin which envelopes the tentacula appears to possess still 
greater sensibility than that of the rest of the body. It is 
more fine, less viscous, perhaps, but in particular more 
nervous. . 
With respect to the function of the tentacula, it is ques- 
tionable if it be confined to this general sensibility, and 
