In Buccinam and other syphoniferous ge-• nera, the structure of the proboscis is much more complicated and curious, (fig. 182.) " The proboscis, which carries with it the oesophagus in its different states of protrusion, is organised with wonderful artifice, being not only capable of flexion in every direction com bined with limited power of retraction or elongation, but it can be entirely lodged in the interior of the body, folded within itself, so that that half which is nearest the base en closes the other portion : from this position it is protruded by unfolding itself like the finger of a glove or the tentacle of a snail, only it is never completely inverted. We may repre sent it as composed of two flexible cylinders (fig. 182, a, b,) one inclosed within the other, the upper borders of which join, so that by drawing outwards the inner cylinder, it is elongated at the expense of the other, and on the contrary, by pushing it back, the internal cylinder becomes lengthened by its shortening. These cylinders are acted upon by a number of longitudinal muscles (c, c), all very much divided at each extremity, the internal or su perior divisions being fixed to the parietes of the body, whilst at the other end they are attached to the inner wall of the internal tube (a) of the proboscis, along its whole length, extending even to its extremity ; their action is obviously,to draw the inner cylinder, and con scquently the entire proboscis inward. This being done, a great part of the inner sur face of the inner cy linder becomes a part of the external surface of the outer cylinder, whilst the contrary oc curs when the pro boscis is elongated and protruded.
The elongation of the inner cylinder by the unfolding of the outer, or what is the same thing, the pro trusion of the probos cis, is effected by the intrinsic annular mus cles which assist in forming the organ ; they surround it throughout its whole length, and by their suc cessive contractions force it outwards; one espe cially, seen at b, placed near the junction of the extremity of the outer cylinder with the inte guments of the head, which is stronger than the rest. When the proboscis is protruded, its retractor muscles acting separately, bend it in every direction, being in this case antago nists to each other. The internal cylinder incloses the tongue (f), the salivary canals (e), and the greater part of the esophagus (d), but its principal use is to apply the extremity of the tongue to such objects as the animal would suck or erode by its armed surface.
In Aplysia, Akera, and others, the mouth consists of a fleshy mass of considerable strength, to which are attached muscular bands proceeding from the sides of the body, serving for its movements, some drawing it forwards whilst others retract it, but there are no jaws nor anything equivalent to them, except the cartilaginous hardness of the lips.
But in such of the Gasteropoda as devour vegetable matter, the mouth, instead of being a proboscis, consists of a strong muscular cavity, inclosing a dental apparatus adapted to the division of the food. In the Snail, Slug, Limneus, Planorbis, &c., this is a single cres cent-shaped horny tooth, attached to the upper surface, and furnished along its opposite edge with sharp points, separated by semicircular cutting spaces, admirably adapted for the di vision of vegetable food.
The dental organs of Tritonia and Seyllcea are, however, still more perfectly contrived for such a purpose. The muscular mass of the mouth is strong and powerful, but instead of the single tooth of the Snail, it is armed with two cutting blades (fig. 183, b, b), horny in their texture and exceedingly sharp, resembling in every respect a pair of strong curved shears, from which in fact they only differ in the mode of their union, the spring of the one being replaced by an articulation (c) inclosed in a synovial capsule. These blades are approx imated by strong muscular fibres, and few animal structures can resist their edge. The lips (h), which are placed in front of these teeth, are strong and very flexible, forming a muscular tube, by means of which the food is•seized and brought within the power of its formidable jaws, and then the divided morsels, being seized by the horny teeth which invest the tongue (d), are conveyed into the (esopha gus.
Tongue.—The tongue in these Mollusca is generally a very important organ, serving not only as a necessary auxiliary in deglutition, but often as a means of eroding the food : in fact, in one tribe only, Thethys, is it found to be deficient. In most of the proboscidean spe cies the tongue is short, and covered with sharp, horny, and recurved spines, which, seizing the morsels of food taken into the mouth by a sort of peristaltic motion, push it backwards into the esophagus. To some ge nera which have no proboscis, the tongue is of extraordinary length ; thus in Ballot's it is half as long as the body, and io Patella, Turbo, Pia', and others, it much exceeds in length the entire animal. The tongue of Pa tella, which is three times the length of the body, is represented 184 ; it is supported by two cartilaginous pieces (b, b) placed on each side of its root; from these arise strong and short muscular bands, which wield the or gan. The surface of this singular tongue, a magnified view of which is given at B, is armed with minute though strong teeth, placed in transverse rows and arranged in three series ; each central group consists of four spines, while those on the sides con tain but two a-piece. It is only at its an terior extremity, however, that the tongue so armed presents that horny hardness needful for the performance of its functions, the posterior part being comparatively soft ; it would seem, therefore, that in proportion as the anterior part is worn away, the parts behind it assume gradually the necessary firmness and advance to supply its place. The action of this curious instrument is as follows :—in the upper part of the circumference of the mouth we find a semicircular horny plate, resembling an upper jaw, and the tongue, by triturating the food against this, gradually reduces substances how ever hard. On opening the Patella, the tongue is found doubled upon itself, and folded in a spiral manner beneath the viscera.