| Fig. 1.—Heterocercal Tail of Acipenser. a, Modified median scales (“fulcra”); b, bony plates. |
| From Cambridge Natural History, vol. vii., “Fishes, &c.,” by permission of Messrs. Macmillan & Co., Ltd. |
| Fig. 2.—Cladoselache. (After Dean.) |
| From “Challenger” Reports Zool., published by H.M. Stationery Office. |
| Fig. 3.—Chlamydoselachus. (After Günther.) |
The median fins are to be regarded as the more primitive. The fundamental structure of the vertebrate, with its median skeletal axis and its great muscular mass divided into segments along each side of the body, indicates that its primitive method of movement was by waves of lateral flexure, as seen in an Amphioxus, a cyclostome or an eel. The system of median fins consists in the first instance of a continuous fin-fold extending round the posterior end of the body—as persists even in the adult in the existing Dipneusti. A continuous median fin-fold occurs also in various Teleosts (many deep-sea Teleosts, eels, &c.), though the highly specialized features in other respects make it probable that we have here to do with a secondary return to a condition like the primitive one. In the process of segmentation of the originally continuous fin-fold we notice first of all a separation of and an increase in size of that portion of the fin which from its position at the tip of the tail region is in the most advantageous position for producing movements of the body. There is thus formed the caudal fin. In this region there is a greatly increased size of the fin-fold—both dorsally and ventrally. There is further developed a highly characteristic asymmetry. In the original symmetrical or protocercal (= diphycercal) type of tail (as seen in a cyclostome, a Dipnoan and in most fish embryos) the skeletal axis of the body runs straight out to its tip—the tail fold being equally developed above and below the axis. In the highly developed caudal fin of the majority of fishes, however, the fin-fold is developed to a much greater extent on the ventral side, and correlated with this the skeletal axis is turned upwards as in the heterocercal tail of sharks and sturgeons. The highest stage in this evolution of the caudal fin is seen in the Teleostean fishes, where the ventral tail-fold becomes developed to such an extent as to produce a secondarily symmetrical appearance (homocercal tail, fig. 4).
The sharks have been referred to as possessing heterocercal tails, but, though this is true of the majority, within the limits of the group all three types of tail-fin occur, from the protocercal tail of the fossil Pleuracanthids and the living Chlamydoselachus to the highly developed, practically homocercal tail of the ancient Cladoselache(fig. 2).
The praecaudal portion of the fin-fold on the dorsal side of the body becomes broken into numerous finlets in living Crossopterygians, while in other fishes it disappears throughout part of its length, leaving only one, two or three enlarged portions—the dorsal fins (fig. 4, d.f.). Similarly the praecaudal part of the fin-fold ventrally becomes reduced to a single anal fin (a.f.), occasionally continued backwards by a series of finlets (Scombridae). In the sucker-fishes (Remora, Eckeneis) the anterior dorsal fin is metamorphosed into a sucker by which the creature attaches itself to larger fishes, turtles, &c.
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| From Cambridge Natural History, vol. vii., “Fishes, &c.,” by permission of Messrs.Macmillan & Co., Ltd. | |
| Fig. 4.—Tilapia dolloi, a teleostean fish, to illustrate externalfeatures. (After Boulenger.) | |
A, Side view. B, First branchial arch. a.f, Anal fin. c.f, Caudal fin. d.f, Dorsal fin. g.f, Gill lamellae. | g.r, Gill rakers. l.l, Lateral line organs. n, Nasal opening. p.f, Pelvic fin. p.op, Preoperculum. pt.f, Pectoral fin. |
The paired fins—though more recent developments than the median—are yet of very great morphological interest, as in them we are compelled to recognize the homologues of the paired limbs of the higher vertebrates. We accordingly distinguish the two pairs of fins as pectoral or anterior and pelvic (= “ventral”) or posterior. There are two main types of paired fin—the archipterygial type, a paddle-like structure supported by a jointed axis which bears lateral rays and exists in an unmodified form in Neoceratodus alone amongst living fishes, and the actinopterygial type, supported by fine raylike structures as seen in the fins of any ordinary fish. The relatively less efficiency of the archipterygium and its predominance amongst the more ancient forms of fishes point to its being the more archaic of these two types.
In the less highly specialized groups of fishes the pectoral fins are close behind the head, the pelvic fins in the region of the cloacal opening. In the more specialized forms the pelvic fins frequently show a more or less extensive shifting towards the head, so that their position is described as thoracic (fig. 4) or jugular (Gadus—cod, haddock, &c., fig. 5).
| Fig. 5.—Burbot (Lota vulgaris), with jugular ventral fins. |