Alimentary canal of Buccinum.

Fig, 274. Alimentary canal of Buccinum.

(1450). But although in Buccinum, Pterotrachea, and kindred genera the stomach is thus devoid of complication, it is by no means unfre-quently found to be provided with a powerful crushing apparatus, that forms a strong gizzard, adapted to bruise, cut, or tear the food introduced into it. In Scyllcea, for example, this gizzard, situated at the entrance to the stomach, contains twelve horny cutting blades, disposed around its interior and arranged in a longitudinal direction; their sharp edges therefore, meeting in the centre, efficiently divide whatever passes between them towards the proper digestive stomach. In Aplysia there is first a capacious crop, then a strong gizzard studded internally with pyramidal blunt teeth; and to this succeeds a third cavity armed with sharp-pointed hooks attached to one side of its walls, and so disposed as to form a kind of carding-machine, by which the food is still more effectually torn to pieces.

(1451). Various modifications in the form and structure of these stomachal teeth are met with in the different genera of the Gasteropoda that possess such an apparatus; but whatever their shape, size, number, or position, the office assigned to them is the same.

(1452). The liver is proportionately of very large size in the Mollusca we are now describing. Its composition is similar in all, being made up of bunches of secreting follicles united by the branches of their excretory ducts, and kept together by means of a delicate cellulosity and the ramifications of blood-vessels. We have already described the hepatic viscera of the Snail; and the liver of Buccinum, unravelled so as to show its intimate structure, is represented in the preceding figure (fig. 274, n, o, p), which requires no additional explanation.

(1453). But if the structure of the liver is similar in all the Gaste-ropod Mollusca, the manner in which the bile is poured into the intestine varies remarkably. The most ordinary position of the orifices of the hepatic ducts is at the termination of the stomach, in the vicinity of the pylorus, as is the case in the majority of other animals; but many exceptions to this rule are met with in the class before us.

(1454). In Scyllcea the bile is poured into the oesophagus just before it terminates in the gizzard; in many genera the biliary canals open into the stomach itself; and in one remarkable genus, Onchidium, there are three distinct livers, each provided with its proper excretory duct, and, what is still more anomalous, these three glands, which in every particular strictly resemble each other, unless perhaps in size, pour the secretion that they furnish into three different situations, - the first into the oesophagus; the second into the oesophagus likewise, and the third into the gizzard, which forms the first of three stomachal cavities.

(1455). In Doris, a figure of which is given above, a still more extraordinary arrangement is met with. One set of ducts derived from the liver penetrate the stomach, and pour the bile into that cavity, while another large canal, equally given off from the liver, terminates at the exterior of the body by an orifice situated in the vicinity of the anus (fig. 266); and thus a part of the bile secreted would seem to be expelled from the system as excrementitious matter - a fact of no ordinary importance to the physiologist, as it would itself go far to prove that the function of the liver is not merely limited to the supply of a secretion of importance in the digestion of food, but that it powerfully cooperates with the respiratory system in purifying the circulating fluids by decarbonizing the blood.

(1456). Other secretions, apparently of an excrementitious character, are furnished by many Gasteropods. Thus, in Aplysia a glandular mass is imbedded in the opercular flap that protects the gills, from which, at the pleasure of the animal, a reddish liquor is made to exude in sufficient abundance to obscure the water around it, and thus conceal it from pursuit. Another gland furnishes an acrid limpid fluid, that distils from an orifice near the oviduct; but the use of this last secretion is as yet unknown.

(1457). The scattered condition of the nervous ganglia, characteristic of the Heteeogangliata, is well exhibited in the Pectinibranchiate Gasteropods, more especially as it not unfrequently happens that the ganglionic centres themselves are of an orange or reddish colour, while the nerves derived from them present their usual appearance.

(1458). In Buccinum the brain still occupies its usual position above the oesophagus (fig. 275, d), and gives off nerves to the organs of sensation, and large twigs (c c) to the eminently sensitive proboscis. A large nervous mass placed beneath the oesophagus (i) is connected with the former by several communicating nerves that embrace the oesophageal tube. Other ganglia, of smaller size (k, I, n), are distributed in distant parts of the body, and supply the viscera to which they are contiguous, whilst they are connected among themselves, and with the brain, by nervous cords passing from one to another.

(1459). In Pterotrachea the same dispersion of the central ganglia of the nervous system is equally evident. The brain and nervous collar around the oesophagus occupy their usual situation, and give nerves to the tentacles, eyes, and parts around the mouth; while four smaller ganglia (fig. 269, i) are placed in the immediate vicinity of the foot, to which and to the neighbouring viscera they distribute their branches.

(1460). But in the most elevated Gasteropods the ganglia assume greater concentration, and the brain exhibits much larger dimensions as compared with the size of the body. Thus, in the Snail (fig. 265) we find only two great nervous masses - the brain (Z) (a large ganglion placed above the oesophagus, and supplying the nerves connected with sensation), and an equally large subcesophageal mass (m), whence proceed nerves to all the viscera and locomotive organs. Here, therefore, we have another example of the great law that we have already so often illustrated - the diminution in number and the increase in size of the nervous centres as we rise from lower to more exalted types of animal organization.