(1437). These two blades are very sharp, and there is nothing that has life that they cannot cut when the animal causes the cutting edges to glide over each other. For this purpose muscles of great strength are provided, the fibres of which are transverse; and their office is to approximate the two blades, that are again separated by the natural elasticity of the articulation whereby they are united at one extremity.

(1438). The aliment, once cut by the jaws, is immediately seized by the papillae of the tongue, which, being sharp and directed backwards, continually drag, by a kind of peristaltic movement, the alimentary materials into the oesophagus.

(1439). The fourth and most complicated form of mouth is found in the Pectinibranchiate Gasteropods; and with its assistance these animals can bore through the hardest shells in search of food, making a hole as round and smooth as if it had been made by a drill of human contrivance. It is from Cuvierwe again borrow the subjoined description of this unique apparatus*.

Mouth of Tritonia Hombergii.

Fig. 272. Mouth of Tritonia Hombergii.

* Memoire sur le Tritonia.

(1440). The proboscis of Buccinum is organized with marvellous artifice. It is not simply provided, like that of the elephant, with the means of flexion and extension, joined with a limited power of contraction and elongation, but it can be entirely retracted into the body by drawing itself into itself in such a manner that the half of it which forms its base contains and encloses the half nearest its point; and it can protrude itself from its sheath thus formed, by unrolling itself like the finger of a glove, or like the horns of the garden snail: only it is never completely retracted, but always remains more or less folded upon itself.

(1441). It may be represented as being composed of two flexible cylinders, one contained within the other, as shown in the annexed figure (fig. 273), the upper edges (i i) of the two cylinders being continuous in such a manner that by drawing out the inner cylinder (bb) it becomes elongated at the expense of the other, and on pushing it in again it becomes shorter, while the outer cylinder (k) is lengthened by adding to its upper margin.

(1442). The reader must now imagine a multitude of longitudinal muscles (d d), all very much divided at both their extremities, and attached by one end to the parietes of the body, whilst by the opposite they are fixed to the interior of the inner cylinder of the proboscis (b) along its entire length and as far as -its extremity. It is evident that the action of these muscles will retract this cylinder, and consequently the entire proboscis, into the body.

(1443). When thus retracted, a great part of the inner surface of the internal cylinder (6) will necessarily become a portion of the external surface of the outer cylinder (k), and the contrary when the proboscis is protruded. It is in consequence of this that the insertions of the muscles (d d) vary in position.

(1444). The protrusion of this proboscis is effected by the action of the circular muscles that form its walls.

(1445). When the proboscis is extended, the retractor muscles (fig. 273, d d), if they do not act altogether, serve to bend it in any direction, thus becoming the antagonists to each other.

Proboscis of Buccinum.

Fig. 273. Proboscis of Buccinum.

* "Memoire sur le grand Buccin (Buccinum undatum), et sur son Anatomie".

(1446). In the internal cylinder are contained the tongue, with all its apparatus (e e), the salivary ducts (f), and the greater part of the oesophagus (g); but the principal use of the proboscis is to apply the end of the tongue to the surface of bodies that the Buccinum wishes to erode and suck. The tongue itself (e) is a cartilaginous membrane, armed with hooked and very sharp spines. It is sustained by two long cartilages, the extremities of which form two lips (c), that can be separated or approximated; or the cartilages can be made to move upon each other by the mass of muscles in which they are imbedded. When these cartilages move, the spines that cover the tongue are alternately depressed and elevated; and by a repetition of similar movements, aided perhaps by some solvent quality in the saliva, the hardest shells are soon perforated by this singular file.

(1447). The salivary glands are lodged in the visceral cavity, and are composed of numerous secerning caeca enclosed in a membranous capsule (fig. 274, h, k): their ducts (g, e), which are necessarily as long as the proboscis when extended to the utmost, open by two apertures placed at the sides of the spinous tongue (b.) The oesophagus (fig. 273, g g) runs along the centre of the proboscis throughout its entire length, and, when that organ is protruded, becomes nearly straight; but when the proboscis is drawn in, the oesophagus is folded upon itself among the viscera.

(1448). Just at the commencement of the stomach there is a small crop (fig. 274, f); and the stomach itself is single, without anything in its texture requiring special notice, - its lining membrane being soft, and gathered into longitudinal folds (i).

(1449). Equally simple is the alimentary apparatus of the Heteropoda. In these the stomach (fig. 269,f) is a mere dilatation of an intestiniform tube. The intestine is not lodged in the general cavity of the body, but, with the mass of the liver, is contained in a kind of bag attached to the back of these singularly-formed animals, and in some genera, as for example Carinaria, is defended by a delicate transparent shell, which in appearance offers a miniature resemblance to the Argonaut. It is in this visceral sac that the heart and generative apparatus are likewise generally enclosed; but in many forms of the Heteropoda both the appended sacculus and shell are wanting, in which case the viscera are, of course, lodged in the general cavity of the body.