We have already observed that the true fishes, to which we must now direct our attention, differ from the round-mouths in several important characters. They possess two pairs of extremities, the pectoral and pelvic fins; they have a pair of nostrils; there is also a well-developed skull, which includes a series of cartilaginous or bony arches situated in the wall of the gut and between the successive gill clefts. These branchial arches bear a certain resemblance to the basket-work arrangement of cartilage in the round-mouths, but for various reasons are not regarded as having been derived from the latter. It is from the first pair of these arches that the jaws are formed, organs which make their first appearance in the lower fishes. The skeleton shows great development in other directions. The notochord is present in its primitive condition during the earlier stages of development, but it becomes surrounded, and in many cases largely suppressed, by the portions of the vertebræ. Each vertebra consists of an upper and a lower portion, the upper forming an arch round the nerve cord and the lower bearing lateral processes or ribs. In the higher forms the two portions become united round the notochord, and the resulting vertebra may encroach inwards until it becomes solid, the notochord then remaining only as a series of small pieces of cartilage between the successive units of the vertebral column. There is also, of course, a skeleton in connection with the limbs, but this does not yet correspond in detail to that of the other classes of vertebrates. The brain is much more highly developed than in the round-mouths; in many forms, particularly, there is a considerable development of the cerebral hemispheres, a portion of the fore brain, and the seat of the higher intelligence. The eyes and ears show the same main features as in the higher groups. The ear has three semicircular canals, the same number as in man, as against two in the lamprey and one in the hag. Fishes are possessed of a peculiar 'sixth sense,' the organs for which are situated in two lines running along the sides of the body, the latter forming a familiar feature of a cod or whiting. The nature of this sense is not definitely known, but it appears to be of the nature of a very refined appreciation of wave motions in the water. It is probably by means of these 'lateral line' sense organs, for instance, that fishes are so easily able to avoid obstacles when swimming in the dark.
Fig. 70.—Diagrammatic dissection of dog-fish (Scyllium).
sk, Skull; gh, brain; n, nerve cord; ch, notochord; ho, reproductive gland; un, kidney; wk, vertebræ; k, jaw; z, tongue; ks, gill slits; ar, arteries; ph, pharynx; h, heart; lb, liver; m, stomach; mil, spleen; pan, pancreas; vd, intestine, with spiral fold; cöl, body cavity; r, rectum.
The heart has one auricle and one ventricle, except in a single group which we shall afterwards mention. The heart is situated immediately behind the gills, to which the blood is pumped directly by the ventricle. From the gills, the blood is collected and distributed throughout the body, is re-collected and returned to the auricle. The circulatory system is provided with a set of blood-glands, essentially similar to those in man himself. There is a spleen, a thymus and a thyroid gland, and a pair of suprarenal bodies. The several functions of these glands form an extremely difficult chapter of physiology, but, broadly speaking, they are concerned in the formation of the white blood corpuscles, the removal of worn-out red corpuscles, and in certain obscure but important chemical changes in the composition of the blood. The blood itself consists of a fluid plasma in which float white and red blood corpuscles, the latter being flat and oval, and containing the same oxygen-carrying substance, hæmoglobin, as is found in mammals.
The alimentary canal is simple. The mouth cavity is succeeded by the pharynx, the walls of which are perforated by the gill clefts. Next follow the gullet, the stomach, and the intestine, the division into the three portions being apparent often only after close examination. There are generally gastric glands, of simple form, a large liver, and almost always a pancreas. The kidneys and the reproductive organs open to the exterior by a common duct. A further characteristic feature of the fishes is their external covering of scales. True teeth, comparable to those of the higher vertebrates, appear first in this group. Some of the main features that we have mentioned are illustrated in Fig. 70.
Careful study of the fishes makes it evident that they have very much in common with the higher groups of vertebrates. It is not too much to say, with Haeckel, that there is far more difference between Amphioxus and the fishes than between the fishes and man.
There are four main divisions of the fish group. The first, that of the Elasmobranchs, comprises the sharks and dog-fishes, the skates and the rays. The second group, the Ganoids, includes the sturgeon and a few less well-known forms. The third, the so-called bony or food fishes, includes the vast majority of ordinary species, such as the salmon and trout, the cod, herring, eel, and all our ordinary freshwater species. The fourth, the 'lung fishes,' consists of three very remarkable species, which we shall later describe in detail. The mutual relationships of these groups is well understood, and it is possible to make fairly definite statements regarding their evolution.
The Elasmobranchs are at once the most primitive and, so far as is known, the oldest of the four. From these evolved the lower Ganoids, which then divided into two main branches, the first of which led up to the higher Ganoids and through them, at a comparatively late date, to the bony fishes. The second led to the lung fishes and, either through them or along a somewhat parallel line, to the amphibians and the land vertebrates generally. It is with the second line, therefore, that we shall be mainly concerned.