FINS oF FISHEs. Fishes' fins may consist of mere folds of the skin, or these membranous folds may he supported by cartilaginous or bony rods, the fin rays. When the supporting rays arc unsegmented, in which ease they are usually strong, we have a spiny-rayed fin like the first dorsal of the perch ; the whole or a part of a given fin may be spiny-rayed. Such fishes are elassed as `aeanthopterons! When the rays are segmented, we have soft-rayed fins, and the fishes possessing only such are classed as 'main eopterous.' Fins arise as folds of the skin. In young fishes these folds are much more exten sive, and later disappear except in the region where the pornmnent fins are to develop. For the wholly different tins of certain ancient fishes and the few existing' lung-fislips, see DirNol.
Fins are of two kinds—paired and unpaired. paired fins, placed at or near the ventral side of the body, are the pectoral and anal, corre sponding to the anterior and posterior limbs respectively of higher vertebrates. Along the median dorsal line we may have one or more unpaired fins—the dorsal tins. The caudal fin terminates the body poste riorly. The anal fin (usual
ly one, but sometimes several) is the unpaired fin in the median ventral line of the body, posterior to the anus. In flounders and in eortain fish embryos there is a continuous dorsal and ventral told of skin supported by lin-rays. In most adult fishes only iso lated patches of the con tinuous fin remain—two dorsal, one ventral, and one caudal. See illustrations under FISIL.
Fins may he variously modified. The pectorals may be greatly broadened and lengthened, and act as flying organs, as in the flying fishes. The ventral fins may be entirely absent, as in the Apodes. They may be united in a manner to produce a sucking disk, as in the lumpsueker, or the dorsal fin may be transformed into a sucking disk, as in the remora. The auals may be entirely wanting, as in certain sharks. The modifications of the caudal fin (tail) fall into two forms, proposed by Agassiz, which are char acteristic of groups and much used in the classifi cation of fishes. These forms are: