The salivary glands are constantly present and seem to present a size and importance corresponding with the mode in which the mastication of the food is accomplished. In those genera which are provided with a cutting apparatus placed in the mouth, they are very largely developed, as also in most of the ptoboscidean species, and it is only in the Cyclobranchiate order, where the long spiral tongue is used rather for the abrasion than the mastication of the food, that they become small, and, in a very few instances, undistinguishable. In fig. 190, which represents the viscera of the Snail, these glands are marked with the letters a, and this engraving will give a good idea of the general structure which they present, and of the ordinary termination of the ducts which pour the saliva into the oral cavity. The glands are placed along the sides of the stomach, which they partially invest, and sometimes those of the opposite sides are intimately united with each other; their colour is whitish and semi-transparent, and they are formed of small lobes, which, in many species where their texture is less compact, may be distinctly seen to be formed of the ramifications of their arborescent ducts, each ultimate division of which is terminated by a secreting granule. In Vaginulus (fig. 189) the salivary glands are small, but in addition to the ordinary struc ture (f) we find an additional tube or slender ccecum (*if), which, lying at first upon the stomach, passes through the nervous collar to join the duct by which the saliva is discharged. The secondary divisions of the ducts gradually unite to form an excretory canal for each of the two glands, which invariably pour the salivary secretion into the mouth in the vicinity of the tongue. When very small, as in Testacella, Onchidium, and Ilaliotis, they are found to be merely arborescent tufts placed on each side of the oral mass. In all the Pectinibranchiate order, where the mouth is converted into a protrusible proboscis, the glands themselves (fig. 182, i) are found within the visceral cavity, and their ducts (e, e) are very long and tortuous so as to follow the movements of the proboscis in which they are lodged, running in contact with the esophagus to open at the extremity of that tube on each side of the spiny tongue; it is even probable that the secretion which they furnish at that point may assist, in some mea sure, in the destruction of the shells and other hard bodies, which are submitted to the con tinued action of this organ.
In Doris and Pkurobranchus a glandular structure of considerable size is found near the commencement of the cesophagns, which is of a brownish colour and plentifully furnished with bloodvessels. This has been looked upon as an auxiliary salivary organ, but as its duct has not been as yet satisfactorily traced, its real nature is unknown : but in Janthina there are distinctly four salivary glands, each furnishing a distinct duct ; two of these run, as in Bucci num, to the extremity of the proboscis, whilst the other pair empties the secretion of the corresponding glands into the commencement of the oesophagus.
Biliary system.—The liver throughout the whole class is of great comparative size, en veloping the convolutions of the intestines and filling a large portion of the visceral cavity. That of the Snail consists of four large lobes (fig. 190, h), each divisible into lobules, and these again into secreting granules, from each of which issues an excretory duct. The ducts gradually unite into larger trunks, so that the whole organ, when unfolded, accurately repre sents a bunch of grapes, the stem of which would be the common biliary duct. In the same ani mal the excretory ducts from each of the divisions of the liver unite into one canal, which opens into the pyloric extremity of the stomach (g) in such a manner that as much bile must be poured into the stomach itself as into the com mencement of the intestine. In the Slug the
liver consists of five lobes, and from these are derived two distinct biliary canals, which open separately into the intestine, one on each side of the pylorus. A similar disposition occurs in Vaginalas (fig. 1E19, I, it').
In Scyllaa the liver (fig. 187, d) is divided into six small and detached round masses, the excretory duets of which open above the point where the esophagus joins the singularly armed gizzard (e). The liver of Aplysia is very large and forms three principal masses, among which are seen the convolutions of the intestine. The biliary canals are very wide and open into the third stomach near the aperture communicating with the rudimentary pancreas (fig. 188, c). In Testacella Haliatoidea there are two livers perfectly distinct from each other, and from each arises a proper duct, which opens sepa rately into the commencement of the intestine near its origin. Onchidium furnishes us with a still more curious arrangement, being pro vided with three distinct livers, pouring their secretions by separate canals into differentparts of the alimentary tube. Each portion perfectly resembles the others in external appearance, rind in structure as well as in the nature of their respective secretions. The excretory eanal which proceeds from the largest mass enters the esophagus, discharging itself near to its cardiac termination ; the duet of the second terminates near the same point, whilst the bile produced by the third is poured into the gizzard itself. The insertion of the two former above the gizzard would seem intended for the same purpose as the abundant secretion which is poured into the ventrieulus succenturiatus of Birds, namely, to moisten the food before its in troduction into the gizzard ; it is, however, sin gular to find the biliary fluid employed for this purpose ; nor is the insertion of the third duet into the first of the three stomachs of this animal less extraordinary, a similar arrangement occur ring only in a few fishes, as in the Diodon Mule.
The liver of Doris is very large, and not only is the bile which it secretes discharged by large and numerous ducts into the stomach, so wide, indeed, that it is difficult to conceive how the food is prevented from entering them, but moreover the liver furnishes a second duct of large calibre, which opens externally in the vicinity of the anus. A part of the bile in this case is evidently exerementitious, as there is no doubt that the second canal takes its origin from the substance of the liver. " This," says Cuvier speaking upon this subject, " is the first instance of the kind which I have met with, and the fact was sufficiently singular to make me hesitate long and examine the matter with all possible precaution before admitting it. It is only by one supposition that it can be explained otherwise,—namely, that the lobes of two different glands are so interwoven that they are not to be distinguished from each other, one portion producing bile used in the process of digestion, and the other secreting a fluid which escapes by the canal in question." Before its termination externally, the secondary duet communicates by a short canal with a lateral receptacle, which forms a kind of gall bladder, having its lining membrane much corrugated and its walls apparently muscular ; this is probably a reservoir for the excremen titious fluid, in which it may be retained until the animal feels its discharge necessary. There is reason to suspect that the fluid thus furnished is a colouring matter, used as a means of de fence, and expelled like the ink of the cuttlefish on the approach of danger, but the matter is undecided.