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.
(1776). In most osseous fishes, in addition to the lips (which, even when fleshy, being destitute of proper muscles, would be unable to, retain food in the mouth), there is generally, behind the front teeth in each jaw, a valve formed by a fold of the lining membrane of the mouth, and directed backwards, so as efficiently to prevent the aliment, and more especially the water swallowed for the purpose of respiration, escaping again from the oral orifice*.
(1777). Fishes have no salivary glands, as saliva to them would be entirely useless: their oesophagus (fig. 317,g; fig. 32 7, d) is capacious and, from the circumstance of their having neither neck nor thorax, extremely short; so that the food when seized is conveyed at once into the stomach.
(1778). The stomach itself is generally a wide cul-de-sac (fig. 317, h), the shape and proportionate size of which vary of course in different species. Its walls are most frequently thin, and the lining membrane gathered into large longitudinal folds (fig. 327, e), so as to admit of considerable distention; but occasionally, as for example in the Mullets, its muscular walls are so thick that it might almost deserve the name of gizzard, and in such fishes its power of crushing the food is no doubt considerable.
Fig. 317. Plan of the general arrangement of the viscera in a Fish.
* Cuvier et Valenciennes, Histoire Naturelle des Poissons, p. 367.
(1779). The intestinal canal in the osseous fishes is a simple tube (fig. 317, i), folded in sundry gyrations proportioned to its length; but in the cartilaginous families, such as the Sharks, the Kays, and the Sturgeons, it presents internally a very remarkable arrangement, evidently intended to increase the extent of surface over which the digested aliment may be spread, for the purpose of absorbing its nutritive portions. In these tribes a spiral valve (fig. 327, h) winds in close turns from the pyloric to the anal extremity of the capacious intestine; so that, although externally the intestine appears short in proportion to the size of the animal, its mucous lining is exceedingly extensive.
(1780). In addition to the biliary secretion which we have met with in the lower animals, another system of chylopoietic glands for the first time makes its appearance in the class before us, from which a fluid termed the pancreatic is poured into the intestine. In the osseous fishes this viscus presents the simplest condition of a gland, consisting of simple caeca (fig. 317, n n); sometimes, as in the Perch, only three in number; at others, as for instance in the Salmonidae, extremely numerous. Prom these appendages a glairy fluid, resembling saliva in composition, is abundantly secreted, and becomes mixed with the bile immediately upon its entrance into the intestine.
(1781). In the cartilaginous fishes, such as Sharks and Rays, the pancreas exhibits a more perfect development, and already presents the appearance of a conglomerate gland (fig. 327, f), from which the pancreatic fluid is conveyed into the intestine through a common duct.
(1782). The liver of fishes is proportionately very large, and generally contains abundance of oil. The bile derived from it is received into a gall-bladder (fig. 317, c), from which a duct of variable length in different species conveys it into the intestine, in the immediate vicinity of the pylorus.
(1783). It is in these animals that we for the first time find the biliary secretion separated from venous blood; and consequently they are provided with a new arrangement of the blood-vessels of the abdomen, which they possess in common with the other Vertebrata, forming what is termed by anatomists the system of the vena porta. The veins derived from the stomach, the intestines, and the spleen, which last viscus now makes its appearance, instead of conveying their contents to the heart, plunge into the substance of the liver and there again subdivide into capillary tubes, thus furnishing to the liver abundance of venous blood, from which the hepatic secretion is elaborated.
(1784). The spleen, now for the first time met with in the animal creation, is a highly vascular organ, generally enclosed in the mesentery between two folds of the intestine (fig. 317, m), and evidently, in position, presenting no precise relations with the stomach. It receives a large supply of arterial blood, which becomes converted into venous as it circulates through this organ, and in that state is transmitted to the liver through the portal system of veins.
(1785). Another important addition to the animal economy, peculiar to the Vertebrate division of animals, is the lymphatic or absorben system of vessels, which in Fishes are abundantly distributed through the body, and ramify like a rich network over the walls of the intestine. These pour the materials absorbed from the body, and the products of digestion, into the principal venous trunks, to be mixed up with the circulating blood *.
(1786). The circulation of the blood in Fishes is carried on by the assistance of a heart composed of two cavities only, which receives the vitiated blood after it has circulated through the system, and propels it through the branchiae, where it is exposed to the influence of the oxygen contained in the surrounding medium. After being thus purified, the blood is collected from the respiratory organs by the radicles of the branchial veins; and these latter vessels, by their union, form the aorta.
Fig. 318. A. Heart of Lophius piscatorius. B. Ordinary structure of a Fish's heart. In both drawings, a, represents the vena cava; b, the auricle; c, the ventricle; d, the bulbus arteriosus; and e, the valvular apparatus guarding its commencement.