(1396). Another viscus, called by Cuvier simply "the bladder," is, from the constancy of its occurrence, evidently an organ of importance; and there seems to be little room to doubt that it is intended to be a receptacle for the seminal fluid, analogous in function to the copulatory pouches we have already met with in Insects and some Crustacea. The reservoir in question, which we have called the sjpermatheca (fig. 264, t), is in the Snail placed above the stomach; and the canal derived from it accompanies the sacculated oviduct, which it ultimately joins near its termination, in such a manner that the ova must pass the orifice of its duct as they are expelled from the body. It must nevertheless be confessed that the office here assigned to the "bladder" is rather probable than positively established; for in the Slug, so nearly allied to the Snail in its general organization, the excretory duct of this organ opens into the common generative sac by an aperture distinct from that which leads into the oviduct, although even here the two are closely approximated. Cuvier suggests that perhaps it may furnish some material useful in forming an envelope for the ova; but experiments are still wanting upon this subject.

* Loc. cit.

(1397). There is still another set of organs connected with the canal by which the eggs escape from the oviduct of the Snail; and these, although peculiar to the genus we are examining, no doubt furnish a secretion of importance to their economy. They are called the multifid vesicles (fig. 264, y), and are composed of a series of branched caeca derived from two excretory ducts, by which a milky fluid, secreted by the caeca, is poured into the egg-passage prior to its termination.

(1398). Although it will be convenient to speak in more general terms concerning the nervous system of the Gasteropoda than the examination of a particular species would permit, we deem it necessary, before closing our description of the Snail, to describe with some minuteness the senses possessed by these terrestrial mollusks, and more especially the extraordinary mechanism provided for withdrawing the most important instruments of sensation into the interior of the body when they are not in actual employment.

(1399). The only senses that we can expect to meet with in animals deprived of either an external or internal skeleton are those of taste, smell, vision, and touch.

(1400). The sense of taste, judging from the structure of their tongue, must be extremely obtuse; and although these creatures are evidently possessed of smell, it is not easy to point out where their olfactory apparatus is placed. The eyes, however, are now found to present a perfection of structure correspondent with the enlarged brain, and occupy a singular position, being situated at the extremities of the two superior tentacula appended to the head; while the inferior pair, adapted, as it would seem, more exclusively to the perception of tactile impressions, are deprived of visual organs. Both the upper and lower tentacula are retractile, and can be completely inverted, so as to be withdrawn into the interior of the body. To effect the inversion by which this end is attained, the plan represented in the accompanying figure is had recourse to. Each tentacle is a hollow flexible cylinder, the walls of which are muscular, and composed of circular fibres. When partially retracted, as in the tentacle marked (c) in the figure (fig. 205), the extremity of the organ is drawn inwards, and two cylinders are thus formed, one within the other: if the outer cylinder is elongated, as in protruding the tentacle, it is at the expense of the inner one; and, on the contrary, the inner cylinder, when the organ is retracted, is lengthened as the other becomes shorter. To evert the tentacle the contraction of the circular muscles that form its walls is sufficient, as they can gradually unroll the whole by squeezing out, as it were, the inner portion; but to effect its inversion a special retractor muscle is required. This muscle (g) arises from the general muscular mass composing the foot and retractile apparatus provided for drawing the snail into its shell: the long slip of muscular fibres so derived, accompanied by the optic nerve (f), traverses the interior of the cylindrical tentacle quite to its extremity, where it is attached, and thus, as the reader will easily conceive, is quite competent to cause its inversion. The lower feeler (d) is represented in the figure as partly retracted by the action of its appropriate muscle (k); while the corresponding one (a), being completely turned inside out, is fully withdrawn and securely packed among the viscera.

(1401). One circumstance connected with the contrivance above described cannot but excite attention; and this is, the peculiar arrangement of the tentacular nerves, whereby they are adapted to changes of position so extensive: the optic nerve (f), for example, must not be stretched even when the eye-bearing tentacula are protruded to the uttermost; and in order to provide for this, when the feelers are not extended, the nerves become thrown into close folds (h), and lodged within the cavity of the body.

(1402). From the above somewhat lengthened account of the anatomy of the Snail, the reader will at least have been able to become acquainted with the general features of an organization which is more or less common to all the members of the extensive class under consideration. We must now, however, enter upon a more enlarged survey of the Gasteropoda, and divide them into such groups as will facilitate our further investigations concerning their structure and habits. The most convenient character by which the different orders composing the class are distinguished has been found to be derived from the nature and arrangement of the respiratory apparatus, which of course varies both in construction and position, according to the circumstances under which particular tribes or families are destined to exist.