This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1380). The mouth is situated upon the under part of the head, and, when widely opened, exhibits a cutting instrument of singular contrivance. Attached to the upper part of the muscular cavity that contains the oral apparatus there is a broad horny plate, the lower edge of which is free, very sharp, and slightly curved, forming, in fact, a knife (fig. 261,f), admirably adapted to divide the leaves and soft parts of vegetables when they are pressed by the action of the lips against its cutting edge.
(1381). The floor of the mouth is provided with a small cartilaginous tongue, covered with delicate transverse striae, and so disposed that by its movements it is well calculated to assist in propelling the food into the oesophagus.
(1382). The oesophagus (fig. 264, e') is continued from the muscular cavity (c') that encloses the dental plate, and soon dilates into a wide stomachal receptacle (v, r), the posterior portion of which, when in situ, is imbedded among the viscera contained in the shell; but in the figure all these parts are unfolded and separated from each other. At the termination of the stomach, biliary vessels (c) are inserted, and the intestine commences, - the latter being a simple tube (a, e), intervolved among the masses of the liver, nearly of equal diameter throughout, and presenting internally neither valves nor any other remarkable appearance. Externally the intestine is intimately connected with the lobes of the liver, among which it lies imbedded, by means of a delicate cellulosity and vascular twigs passing from one to the other. The anal aperture (o), when undisturbed by dissection, is placed upon the right side of the neck, in the immediate vicinity of the orifice (fig. 261, e) that leads into the respiratory cavity.
(1383). Two sets of auxiliary glands are subservient to digestion, the salivary and the hepatic, both of which are of considerable size.
(1384). The salivary glands are semitransparent, and of a whitish colour; they form two irregular broad ribands, which extend along the sides of the stomach (fig. 264, v), spreading out so as to embrace a considerable portion of its extent, and they are occasionally joined together by intercommunicating processes. Two ducts, one derived from each gland, run along the sides of the oesophagus, and open into that canal close to the mouth.
(1385). The liver is of large proportionate dimensions, and is made up of four lobes (fig. 264, b, d) of a dark-brown colour, and composed of an infinite number of minute lobules, every one of which produces a biliary vessel; and these, joining continually with each other, form four large hepatic ducts, one proper to every lobe of the liver. The four hepatic ducts ultimately unite into one great central vessel (c), that opens into the alimentary canal in the immediate vicinity of the pyloric extremity of the stomach.
(1386). The genus of Gasteropoda to which the Snail belongs is composed of air-breathing animals; and we must accordingly expect to find these mollusca provided with a respiratory system specially adapted to the mode of life to which they are destined. The mechanism adopted is as follows: - A capacious chamber, of a somewhat triangular form, is found placed beneath the dorsal surface of the body, and separated from the visceral cavity by a broad muscular septum forming its floor. Into this chamber a wide orifice (fig. 261, e), placed upon the right side of the body near the margin of the shell, allows the atmospheric air to enter. The roof of the respiratory cavity is covered with a most intricate arborescence of blood-vessels (rudely sketched in fig. 264, &), in which the blood is freely exposed to the air therein contained; while the muscular floor, performing alternate movements analogous to those of the human diaphragm, continually draws in and expels the air, so as to ensure its constant renewal. The manner in which respiration is effected, and the general disposition of the circulatory apparatus, is therefore briefly this: - The blood derived from all parts of the body is brought to the respiratory chamber by large veins provided for the purpose; arrived there, it is dispersed through the countless ramifications of delicate vessels spread over the entire roof of the breathing cavity, and thus becomes exposed to the purifying influence of oxygen. The renovated blood is then re-collected by the large pulmonary vein (k); and being conveyed to the heart, which is composed of a single auricle (h) that communicates with a strong ventricular cavity (g), it is propelled through the entire arterial system derived from the aorta (f).
(1387). The whole of that part of the body of the Snail which is not permanently covered by the shell is defended by a thick skin, the surface of which is irregularly furrowed, and continually moistened by a viscid secretion that exudes from glands apparently imbedded in the substance of the integument; and the tenacious slime so furnished, if the creature be irritated, is poured forth in astonishing abundance.
(1388). Nevertheless, besides the slimy material thus copiously supplied by the tegumentary glands, there is in the interior of the animal a special apparatus apparently destined to furnish a viscid fluid of a similar character. The gland alluded to, called by Cuvier*, par excellence, "the secerning organ of the viscosity," is in the Snail a triangular viscus (fig. 264, i) placed in immediate contiguity with the pericardium.
* Histoire des Mollusques - Memoire sur la Limace et le Colimacon.
On opening it, it is found to be filled with an infinite number of very-thin laminae that adhere to the walls of its cavity by one of their edges, and become joined to each other as if by communicating branches. The excretory duct of this slime-secretor, which, we may observe, is found to exist in many other genera of Gasteropods, accompanies the rectum to its termination, where it opens externally in the immediate vicinity of the orifice leading into the respiratory chamber.