The heart of fishes is, properly speaking, a branchial or respiratory heart. It consists of two cavities, an auricle and a ventricle (fig. 257), and the course of the circulation is as follows: The venous blood derived from the liver and from the body generally is poured by the vena cava into the auricle (au), and from this it is propelled into the ventricle (v). From the ventricle arises a single aortic arch (the right), and the base of this is usually dilated into a cavity or sinus, called the "bulbus arteriosus" (ad). The arterial bulb is sometimes covered with a special coat of striated muscular fibres, and may be provided with several transverse rows of valves. In these cases, the bulbus acts as a kind of continuation of the ventricle, being capable of rhythmical contractions. The blood is driven by the ventricle through the branchial artery (b) to the gills, through which it is distributed by means of the branchial vessels, the number of which varies (there are three on each side in a few fishes, four in most of the Bony Fishes, five in the Skates and Sharks, and six or seven in the Lampreys). The aerated blood which has passed through the gills is not returned to the heart, but is driven from the branchiae through all parts of the body; the propulsive force necessary for this being derived chiefly from the heart, assisted by the contractions of the voluntary muscles. In some fishes (as in the Eel) the return of the blood to the heart is assisted by a rhythmically contractile dilatation of the caudal vein. The essential peculiarity, then, of the circulation of fishes depends upon this - that the arterialised blood returned from the gills is propelled i through the systemic vessels of the body, without being sent back to the heart.
Fig. 257. - Diagram of the circulatory system in a Fish, the vessels containing venous blood being longitudinally shaded, and those containing arterial blood being cross-shaded. vc Vena cava; vp Vena portae; au Auricle ; v Ventricle; ab Bulbus arteriosus; b Branchial artery; ba One of the divisions of the branchial artery going to the gills, from which proceeds one of the corresponding branchial veins, by the union of which the subvertebral aorta (sa) is formed ; i Intestine ; k Kidney.
The Lancelet (Amphioxus), alone of all fishes, has no special heart, and the circulation is effected by contractile dilatations developed upon several of the blood-vessels. In the Mud-fishes (Lepidosiren) the heart consists of two auricles and a single ventricle. The blood-corpuscles of fishes are nucleated (fig. 245, e), and the blood is red in all except the Amphioxus.
As regards the digestive system of fishes there is not much of peculiar importance. The mouth is usually furnished with a complicated series of teeth, which, in the Bony Fishes, are not only developed upon the jaws proper, but may be also situated upon other bones which enter into the composition of the buccal cavity (such as the palate, the pterygoids, vomer, branchial arches, the glossohyal bone, etc.) The oesophagus is usually short and capacious, and generally opens into a large and well-marked stomach. The pyloric aperture of the stomach is usually furnished with a valve, and behind it there is usually a number (from one to sixty) of blind appendages, termed the "pyloric caeca." These are believed to represent the pancreas, but there may be a recognisable pancreas either alone or in addition to the pyloric caeca. The intestinal canal is a longer or shorter, more or less convoluted tube, the absorbing surface of which, in certain fishes, is largely increased by a spiral reduplicature of the mucous membrane, which winds like a screw in close turns from the pylorus to the anus. The liver is usually large, soft, and oily, and a gall-bladder is almost universally present; but in the Amphioxus the liver is doubtfully represented by a hollow sac-like organ.
The kidneys of fishes are usually of great size, and form two elongated organs, which are situated beneath the spine, and extend along the whole length of the abdominal cavity. The ureters often dilate, and form a species of bladder, the doubtful representative of the allantois.
Whilst the respiration of all fishes is truly aquatic, most of them are, nevertheless, furnished with an organ which has been generally believed to be the homologue of the lungs of the air-breathing Vertebrates. This - the "air" or "swim bladder" - is a sac containing gas, situated beneath the alimentary tube, and often communicating with the gullet by a duct. In the great majority of fishes the functions of the air-bladder are certainly hydrostatic - that is to say, it serves to maintain the necessary accordance between the specific gravity of the fish and that of the surrounding water. In the singular ' Mud-fishes, (as also in a few Bony Fishes), however, it acts as a respiratory organ, and is therefore not only the homologue, but also the analogue, of the lungs of the higher Vertebrates. In most fishes the air-bladder is an elongated sac with a single cavity, but in many cases it is variously subdivided by septa, or it may give off more or less complicated caeca (fig. 258). In the Mud-fishes the air-bladder is composed of two sacs, completely separate from one another, and divided into a number of cellular compartments. The duct (ductus pneumaticus) leading in many fishes from the air-bladder, and opening into the oesophagus, is the homologue of the windpipe (trachea). The air contained in the swim-bladder is composed mainly of nitrogen in most fresh-water fishes, but in the sea-fishes it is mainly made up of oxygen. The fishes which live habitually at the bottom of the sea, such as the Flat-fishes, possess no swim-bladder, and it is much reduced in size in those which live principally at the surface.
The nervous system of fishes is of an inferior type of organisation, the brain being of small size, and consisting mainly of ganglia devoted to the special senses. As regards the special senses, there is one peculiarity which deserves particular notice, and this is the conformation of the nasal sacs. The cavity of the nose is usually double, and is lined by an olfactory membrane, folded so as to form numerous plicae. Anteriorly, the water is admitted into the nasal sacs by a single or double nostril, usually by two apertures; but posteriorly the nasal sacs are closed, and do not communicate with the pharynx by any aperture. The only exceptions to this statement are to be found in the Myxinoids and in the Mud-fishes.
Fig. 258. - Swim-bladder of Corvina trispinosa, showing appended caeca.
The essential portion of the organ of hearing (labyrinth) is present in almost all fishes, but in none is there any direct communication between the ear and the external medium. In some cases, however, there is a communication between the ear and the swim-bladder, thus foreshadowing the Eustachian tube in man.
As regards their reproductive system, fishes are, for the most part, truly oviparous, the ovaries being familiarly known as the "roe," The testes of the male are commonly called the "soft roe," or "milt." The products of the reproductive organs are often set free into the peritoneal cavity, ultimately finding their way to the external medium by means of an abdominal pore (or pores); or they are directly conveyed to the exterior by the proper ducts of the reproductive organs.