The Liver.—Phylogenetically the oldest gland in connexion with the Vertebrate alimentary canal, and in size by far the largest, the liver arises as a caecal outgrowth from the embryonic mesenteron, and in this primitive stage recapitulates a condition which is retained throughout life in Amphioxus. By the subsequent division and branching of this outgrowth the massive compound tubular gland of the adult Fish is eventually formed.
The liver of Fishes (Figs. 153, 154) is very variable in size, shape, colour, and degree of lobulation. Anteriorly, it is usually moulded to the posterior face of the transverse septum between the pericardial and abdominal portions of the coelom, and from thence extends backwards in the abdominal cavity to a varying distance, in some Sharks as far as the cloaca. Externally, the gland is invested by the peritoneum, which extends on to it from the pericardial septum and forms a suspensory fold, and also from the oesophagus and stomach. The shape of the liver usually bears some relation to that of the body, being, for example, longest in the Eels and broadest in the Rays. In the great majority of Fishes the liver is bilobed, consisting of two sub-equal lateral lobes, disposed longitudinally and confluent anteriorly for a portion of their extent. From this normal type there are a few minor variations.[[261]] In Petromyzon, Lepidosteus (Fig. 155, B), and a few Teleosts (e.g. the Gymnodontes, Lophobranchii, and some Salmonidae) the liver is unilobed. In the Myxinoids and in the Dipnoi (e.g. Protopterus), the organ is bilobed, but the small anterior lobe lies immediately in front of the much larger posterior lobe, with the gall-bladder between the two (Fig. 155, A). In some Teleosts (e.g. Scomber), the liver is trilobed. A gall-bladder is invariably present in either the larval or adult Cyclostomata, in the Chrondrostei, Holostei, Crossopterygii and Dipnoi, and generally also in Elasmobranchs and Teleosts. In the Elasmobranchs it is rarely entirely wanting, as in Sphyrna and Pristis, and in the Teleosts in some of the Gurnards (Trigla). The gall-bladder and bile-duct of Petromyzon fluviatilis atrophy after the metamorphosis which follows the larval Ammocoetes stage, but in Petromyzon marinus the duct, although usually absent, is sometimes retained. In the Ammocoetes the epithelium lining the gall-bladder is ciliated. In some Fishes, as, for example, in many Elasmobranchs, the gall-bladder is more or less completely embedded in the substance of the liver; in others, as in most Teleostomi, the organ is quite distinct from the gland (Fig. 154).
A simple arrangement of the ducts from the liver and gall-bladder is that found in the common Dog-Fish (Scyllium canicula). In this Elasmobranch a cystic duct leaves the gall-bladder, and, after receiving several hepatic ducts from the lobes of the liver, becomes the bile-duct and opens into the commencement of the intestine. In the Myxinoids and in the Dipnoi (e.g. Protopterus), there are but two hepatic ducts, one from each lobe of the liver; these unite and then meet the cystic duct to form the bile-duct (Fig. 155, A). The number of hepatic ducts may, however, be considerably increased, as, for example, in the Siluroid Amiurus,[[262]] where 8-10 separate ducts join the cystic duct. In a few instances one of the hepatic ducts opens directly into the intestine, independently of that which unites with the cystic duct in forming the bile-duct. In the Dipnoi (e.g. Protopterus),[[263]] and in some Teleostomi (e.g. Lepidosteus),[[264]] the bile-duct receives the duct from the pancreas before opening into the intestine.
The Pancreas.—In the Cyclostomes (e.g. Petromyzon, Bdellostoma, Myxine) a rudimentary pancreas is apparently present, but the evidence as to its identity is not wholly conclusive. A well-developed pancreas occurs in Elasmobranchs, in at least one of the Dipnoi, and probably in most Teleostomi.[[265]]
In Elasmobranchs the pancreas is a compact structure, uni- or bi-lobed, and entirely distinct from the liver. In Scyllium canicula (Fig. 153), the bilobed gland lies in the angle between the distal limb of the stomach and the adjacent portion of the intestine, and from the smaller of its two lobes the duct issues to pass to its intestinal aperture near the commencement of the spiral valve. In most of the Teleostomi in which its existence has hitherto been recorded, the pancreas is a singularly diffuse gland; and usually a considerable portion, or even the whole of it, is embedded in the substance of the liver, its lobules accompanying the ramifications of the hepatic artery and duct, and the portal vein. The pancreatic duct usually opens into the intestine near the aperture of the bile duct (e.g. Amiurus); sometimes the two ducts open on the apex of a common papilla (e.g. Acipenser and Amia), or by their union form a common duct (e.g. Lepidosteus). Among the Dipnoi a well-developed pancreas is present in Protopterus,[[266]] embedded in the wall of the stomach and intestine, internal to the peritoneal investment of these organs, and extending even into the first fold of the spiral valve. The gland is traversed by fine ductules which unite together and open into the bile-duct just before the latter enters the intestine. In the remaining Dipnoi the existence of a pancreas has yet to be ascertained. Developmentally, the pancreas resembles the liver, and, histologically, is very similar to that of the higher Vertebrates, consisting of terminal glandular alveoli continuous with intermediary tubular portions, and eventually with the finer ductules, which, by their union, form the main efferent duct.
The Pyloric Caeca.—These structures are caecal outgrowths from the intestine, and are situated close to the pyloric extremity of the stomach and the intestinal apertures of the bile and pancreatic ducts. Wholly wanting in the Cyclostomata and Dipnoi, and, unless represented by a pair of caeca opening into the long, tubular, non-valvate anterior portion of the intestine in the Greenland Shark (Laemargus borealis),[[267]] in the Elasmobranchs also, they are very generally present in the Teleostomi, although extremely variable both in number and arrangement in different families. In Amia there is no trace of pyloric caeca. Polypterus has a single short caecum with a thick muscular wall. In Acipenser, Polyodon, and Lepidosteus, on the contrary, pyloric caeca are unusually well developed. In Acipenser the caeca are not only numerous, but are so connected together by connective tissue and blood-vessels, and so invested externally by the peritoneum, as to form a large, compact, gland-like mass, communicating with the intestine by a single wide duct. In Polyodon the organ is essentially similar, but is lobed externally. In Lepidosteus (Fig. 155, B, py.c), the caeca are also very numerous, but relatively short, and, although united into a compact mass, open by four pit-like orifices into the intestinal cavity. In Teleosts the caeca are subject to extraordinary variations in number, size, and arrangement.[[268]] In some families, and even in groups of higher taxonomic value, they are entirely absent, as is the case with the Siluridae, Esocidae, Cyprinodontidae, Labridae, Plectognathi, and Lophobranchii. The "Sand-eel" (Ammodytes) has but a single caecum; the Turbot (Rhombus maximus) two, and other Pleuronectidae three to five; and the Perch (Perca), three (Fig. 160, py.c).
In other Teleosts, on the contrary, these structures are much more numerous. In Labrus labrax there are about 60, in the Whiting (Gadus merlangus) 120, while in the Mackerel (Scomber scombrus) there are no fewer than 191. If few in number the caeca open separately into the intestine, but when numerous, more or fewer of them may unite to form a smaller number of efferent ducts, as in the Whiting, where four such ducts are formed. In some instances, as in the Tunny (Thunnus), the union of the caeca by connective tissue leads to the formation of a compact mass. As regards their arrangement, the caeca may either be disposed in a whorl round the intestine, as in the Whiting, or in a linear series, as in the Salmon (Salmo) and in some of the Clupeidae.
The mucous membrane lining the anterior pyloric caeca is often developed into a network of ridges, limiting crypt-like or tubular depressions; and not infrequently the epithelium is ciliated.
Fig. 160.—The alimentary canal of a Perch (Perca). an, Anus; in, intestine; oes, oesophagus; py, pylorus; py.c, pyloric caeca; st, stomach. (After Wiedersheim.)