Fig. 250. - Os hyoides, branchiostegal rays, and scapular arch of the Perch (after Cuvier). ss Supra-scapula ; s Scapula ; co Coracoid ; cl Supposed representative of the clavicle ; a Glossohyal bone ; b Basihyal; c Urohyal; d Ceratohyal; e Epihyal; f Stylohyal; br Branchial arches ; t Branchiostegal rays.
The limbs of fishes depart considerably from the typical form exhibited in the higher Vertebrates. One or both pairs of limbs may be wanting, but when present the limbs are almost always in the form of fins - that is, of expansions of the integument strengthened by bony or cartilaginous fin-rays. The anterior limbs are known as the pectoral fins, and the posterior as the ventral fins; and they are at once distinguished from the so-called "median" fins by being always disposed in pairs, usually symmetrically. Hence they are often spoken of as the paired fins.
The scapular arch (figs. 250, 251) supporting the pectoral limbs is usually joined to the skull (occipital bone), and consists of the following pieces on each side: I. The supra-scapula (ss); 2. The scapula (s), articulating with the former ; and, 3. The coracoid (co), attached above with the scapula, and united below, by ligament or suture, with the coracoid of the opposite side, thus completing the pectoral arch. Lastly, there is often another bone, sometimes single, but oftener of two pieces, attached to the upper end of the coracoid, and this is believed to represent the collar-bone or clavicle.*
Fig. 251. - Pectoral limbs of Fishes (after Owen). A, Cod (Morrhua vulgaris): B, Angler (Lophius). ss Supra-scapula; s Scapula; co Coracoid ; r Radius; u Ulna ; cc Carpal bones; f Fin-rays, representing the metacarpus and phalanges of the fingers.
These are the views entertained by Owen as to the composition and nature of the pectoral arch of fishes ; but they are dissented from by Mr Parker, one of the greatest living authorities on this subject.
The fore-limb possesses in a modified form most of the bones which are present in the higher Vertebrata. The humerus, or bone of the upper arm, is usually wanting, or it is altogether rudimentary. A radius and ulna (fig. 251, r, u) are usually present, and are followed by a variable number of bones, which represent the carpus, and some of which sometimes articulate directly with the coracoid. The carpus is followed by the "rays" of the fin proper, these representing the metacarpal bones and phalanges. The pectoral fins vary much in size and in other characters. In the Flying Gurnard (Dac-tylopterus), and the true Flying Fish (Exocaetus), the pectorals are enormously developed, and enable the fish to take extensive leaps out of the water.
The hind-limbs or "ventral fins" are wanting in many fishes, and they are less developed and less fixed in position than are the pectoral fins. In the ventral fins no representatives of the tarsus, tibia and fibula, or femur, are ever developed. The rays of the ventral fins - representing the metatarsus and the phalanges of the toes - unite directly with a pelvic arch, which is composed of two sub-triangular bones, united in the middle line and believed to represent the ischia. The imperfect pelvic arch, thus constituted, is never united to the vertebral column in any fish. In those fishes in which the ventral fins are "abdominal" in position (i. e., placed near the hinder end of the body) the pelvic arch is suspended freely amongst the muscles. In those in which the ventral fins are "thoracic" or "jugular" (i.e., placed beneath the pectoral fins, or on the sides of the neck), the pelvic arch is attached to the coracoid bones of the scapular arch, and is therefore wholly removed from its proper vertebra.
Fig. 252. - Outline of a fish (Perca granulata), showing the paired and unpaired fins. p One of the pectoral fins ; v One of the ventral fins ; d First dorsal fin; d' Second dorsal fin; a Anal fin ; c Caudal fin.
In addition to the pectoral and ventral fins - the homologues of the limbs - which may be wanting, fishes are furnished with certain other expansions of the integument, which are "median" in position, and must on no account be confounded with the true "paired " fins. These median fins are variable in number, and in some cases there is but a single fringe running round the posterior extremity of the body. In all cases, however, the median fins are "azygous" - that is to say, they occupy the middle line of the body, and are not symmetrically disposed in pairs. Most commonly, the median fins consist of one, two, or three expansions of the dorsal integument, called the "dorsal fins" (fig. 252, d, d'); one or two on the ventral surface near the anus - the "anal fins" (fig. 252, a); and a broad fin at the extremity of the vertebral column, called the "caudal fin" or tail (c). In all cases, the rays which support the median fins are articulated with the so-called interspinous bones, which have been previously described. Though called "median," from their position in the middle line of the body, and from their being unpaired, the median fins of fishes, as shown by Goodsir and Humphry, are truly to be regarded as formed by the coalescence of two lateral elements in the mesial plane of the body.