Paired fins[46].

As regards the origin of the limbs or paired fins of fishes there are two principal views. One view, that of Gegenbaur, considers that limbs and their girdles are derived from visceral arches which have migrated backwards. The other view, which probably now has the greater number of supporters, considers that the paired fins of fishes are of essentially the same nature as the median fins.

According to Gegenbaur's view[47] the archipterygium of Ceratodus (fig. 20) represents the lowest type of fin; it consists of a central cartilaginous axis bearing a large number of radiale. The dorsal or pre-axial radiale are more numerous than the ventral or postaxial, and at the margin of the fin[48] the cartilaginous endoskeletal radiale are replaced by horny exoskeletal fin-rays.

Fig. 20. Lateral view of the skeleton of Ceratodus miolepis (after Günther).

1. ethmoid.9. pectoral girdle.
2. scleroparietal.10. proximal cartilage of pectoral fin.
3. frontal.11. pectoral fin.
4. cartilaginous posterior part of cranium.12. pelvic girdle.
5. pre-opercular (squamosal).13. pelvic fin.
6. opercular.14. spinal column.
7. suborbital.15. caudal fin (diphycercal).
8. orbit.

It is impossible here to give a full discussion of the rival views, but some of the points which support Gegenbaur's view may be mentioned. The fact that migration of visceral arches has to be assumed is no difficulty, as it is obvious that migration in the opposite direction has taken place in many Teleosteans such as the Cod, whose pelvic fins are attached to the throat in front of the pectorals. If migration did take place, the pelvic fins being older than the pectoral should be the more modified, and this is the case. Again, if the pectoral girdle is a modified branchial arch, it must at some period have carried a gill, and in Protopterus it does bear a vestigial gill.

According to the view more prevalent at the present time, the paired fins have been derived from two continuous folds of skin and their skeletal supports running forward from the anal region along the sides of the body, their character being similar to the fold that gave rise to the median fins. In support of this view it may be argued that the paired and unpaired fins are often identical in structure, and that some Elasmobranch embryos do show a ridge running between the pectoral and pelvic fins. Then from this continuous fold two pairs of smaller folds may have been specialised off, and in each a number of cartilaginous radiale may have been developed. The fin of Cladoselache from the Carboniferous of Ohio apparently illustrates this condition. It consists of certain basal pieces which do not project beyond the body wall and bear a number of unsegmented cartilaginous radiale, which show crowding together and are sometimes bifurcated distally; they extend throughout the whole fin from the body wall to the margin. From this fin the archipterygium might be easily derived by the enlargement of one of the middle radiale and the segmentation and partial fusion of them all.

Whether the archipterygium be a primitive or secondary type of fin, when it is once reached it is easy to derive all the other types from it. The fins of the other living Dipnoi,—Protopterus and Lepidosiren—are simply archipterygia from which the radiale have almost or completely disappeared, leaving only the segmented axes. Archipterygia too are found in the pectoral fins of the Ichthyotomi, but the postaxial radiale are much reduced.

The ichthyopterygium, or type of fin, characteristic of many modern Elasmobranchs such as Scyllium, may have been derived from the archipterygium by the gradual reduction of the rays on the postaxial side of the axis and their condensation on the pre-axial side. The Ichthyotomi such as Xenacanthus show one stage in the reduction of the postaxial rays, and a further stage is seen in the Notidanidae and some other sharks like Scymnus and Acanthias, in which a few postaxial rays still remain. The condensation of the pre-axial rays when further continued leads to one of the rays getting an attachment to the girdle. Thus the fin comes to articulate with the girdle by two basalia or basal pieces; a third attachment is formed in the same way and the three basalia are called respectively pro-, meso-, and meta-pterygia. By some authors the meta-pterygium and by others the meso-pterygium is regarded as homologous with the axis of the archipterygium.