The caudal fin, or tail, of fishes is always set vertically at the extremity of the spine, so as to work from side to side, and it is the chief organ of progression in the fishes. In its vertical position, and in the possession of fin-rays, it differs altogether from the horizontal integumentary expansion which constitutes the tail of the Whales, Dolphin, and Sirenia (Dugong and Manatee). In the form of the tail, fishes exhibit some striking differences. In some of the Bony Fishes and Ganoids, the caudal extremity of the spine is not bent upwards, but divides the caudal fin-rays into two nearly equal portions, and the symmetrical tail-fin thus produced is said to be "diphycercal." In the great majority of the Bony Fishes the tail-fin appears on inspection to be divided into two equal lobes, and it is then said to be "homocercal" (fig. 253, A). This apparent symmetry is due to the fact that the spinal column seems to terminate in the centre of a triangular bony mass, to the free edges of which the fin-rays are symmetrically attached. In reality, however, the unossified notochord is prolonged into the upper lobe of the tail; and as there is a much larger number of fin-rays below the bent-up notochord than above it, the tail is truly unsymmetrical in its fundamental structure. Lastly, in the Elasmobranchii, and most Ganoids, the tail is conspicuously unsymmetrical (fig. 253, B), and is then said to be heterocercal form of tail.
Fig. 253. - A, Sword-fish, showing homocercal tail; B, Sturgeon, showing the "heterocercal." In these cases, the lower lobe of the tail is conspicuously larger than the upper, owing to the disproportionate development of the haemal rays, and the spinal column is prolonged into the upper lobe of the tail.
In a recently published and important memoir, Professor A. Agassiz has shown that in Pleuronectes and various other living Bony Fishes, the tail of the early embryo is rounded, and is symmetrically developed at the hinder end of the straight notochord ("leptocardial stage"). Soon the chorda becomes arched upwards, and there appears the first trace of a separation of the tail-fin into two portions, only one of which is destined to remain permanently. The superior of these two divisions, when both have become fully marked out, surrounds the end of the upturned chorda (fig. 254, a), and it must be regarded as an embryonic structure, since it finally disappears. The inferior of the two divisions, on the other hand, is placed below the embryonic tail, and is ultimately developed into the permanent tail. At first the permanent caudal fin has the appearance of a distinct lobe, which looks like a second anal fin. In process of growth, however, the embryonic caudal becomes thrown more and more upwards, and the rays of the permanent caudal acquire a fan-like arrangement. At the stage figured below (fig. 254) the tail is truly "heterocercal," and is wonderfully similar in appearance to the tail of many Palaeozoic Fishes. Finally, however (fig. 255), the turned-up end of the notochord becomes replaced by the long "uro-style;" the embryonic caudal diminishes in size and disappears; and the permanent caudal increases in size, and is gradually transformed from a ventral into a terminal appendage, the tail-fin thus assuming its permanent "homocercal" form. It would thus appear that the really earliest stage of the tail in the Bony Fishes and Elasmobranchs is the "lepto-cardial" stage, in which the tail is symmetrical and the notochord straight. This stage is in progress of growth superseded by the "heterocercal" condition, which subsists throughout life in the Elasmobranchs. Finally, the heterocercal tail of the young Bony Fish is in the adult succeeded by the permanent "homocercal" or "diphycercal" tail.
Fig. 254. - Tail of young Flounder (Pleuronectes) in its heterocercal stage of development. a Embryonic caudal fin; b Permanent caudal fin, occupying an inferior position; c Bent-up end of the notochord. (After A. Agassiz.)
The process of respiration in all fishes is essentially aquatic, and is carried on by means of branchial plates or tufts devel-oped upon the posterior visceral arches, which are persistent, and do not disappear at the close of embryonic life, as they do in other Vertebrates. In the Lancelet alone, respiration is effected partly by branchial filaments placed round the commencement of the pharynx, and partly by the pharynx itself, which is greatly enlarged, and has its walls perforated by a series of transverse ciliated fissures. The arrangement and structure of the branchiae differ a good deal in the different orders of fishes, and these modifications will be noticed subsequently. In the meanwhile it will be sufficient to give a brief description of the branchial apparatus in one of the bony fishes. In such a fish, the branchiae are connected with the hyoid arch, and are situated in two special chambers, situated one on each side of the neck. The branchiae are carried upon the outer convex sides of what have been already described as the "branchial arches;" that is to say, upon a series of bony arches (figs. 250 and 256) which are connected with the hyoid arch inferiorly, and are united above with the base of the skull. The internal concave sides of the branchial arches are usually furnished with a series of processes, constituting a kind of fringe, the function of which is to prevent foreign substances finding their way amongst the branchiae, and thus interfering with the proper action of the respiratory organs. The branchiae, themselves, usually have the form of a double series of cartilaginous leaflets or laminae. The branchial laminae are flat, elongated, and pointed in shape, and they are covered with a highly vascular mucous membrane, in which the branchial capillaries ramify. The blood circulates through the branchial laminae, and is here subjected to the action of aerated water, whereby it is oxygenated. The water is constantly taken in at the mouth by a movement analogous to swallowing, and it gains admission to the branchial chambers by means of a series of clefts or slits, the "branchial fissures," which are situated on both sides of the pharynx. Having passed over the gills, the deoxygenated water makes its escape posteriorly by an aperture called the "gill-slit" or "opercular aperture," one of which is situated on each side of the neck. As we have seen before, the gill-slit is closed in front by a chain of flat bones collectively constituting the "gill-cover," or "operculum;" and the gill-covers are finally completed by a variable number of bony spines - the "branchiostegal rays" - which articulate with the hyoid arch, and support a membrane - the "branchiostegal membrane."
Fig. 255. - Tail of adult Flounder. (After A. Agassiz.) v Vertebral column; n Turned-up end of the notochord; h Hypural bones.
Fig. 256. - Gills and heart of the Perch exposed by the removal of the gill-cover on the left side. a First of the four bony arches which carry the gills (b b); b' The lower edges of the gills on the right side; h Heart. (After Van der Hoeven.)