. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 62 4 Biology of the Vertebrates and d), that is, the upturned end of the skeletal axis becomes reduced while the dorsal flange again equals the ventral part. The diphycercal tail of living dipnoans is symmetrical externally while the skeletal axis is shortened so much that the upturned part, present in ancestors of these fishes, has been obliterated (Fig. 553e). In long-bodied fishes like squirming eels, the unnecessary tail fin becomes quite degenerate. F
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 62 4 Biology of the Vertebrates and d), that is, the upturned end of the skeletal axis becomes reduced while the dorsal flange again equals the ventral part. The diphycercal tail of living dipnoans is symmetrical externally while the skeletal axis is shortened so much that the upturned part, present in ancestors of these fishes, has been obliterated (Fig. 553e). In long-bodied fishes like squirming eels, the unnecessary tail fin becomes quite degenerate. Frequently the caudal fin becomes deeply incised, as in the mackerel, Scomber (Fig. 553d), or more or less rounded, as in the "mummichug," Fundulus (Fig. 553c).. C D \^ - E Fig. 553. Different types of tails in fishes, a, protocercal; b, heter- ocercal; c, D, homocercal; e, diphycercal. Since all fins must have flexibility, this end is best accomplished by each fin acting as a whole, without the levers and joints that characterize the locomotor appendages of land vertebrates. To bring this about they are braced in position by skeletal elements which lie between the two walls of the fin fold. The most proximal of these supporting elements (basalia) connect with the girdle or, in the case of median fins, with the neural spines of the underlying vertebrae secondarily. The more distal elements (radi- alia), which may be either cartilage or bone, splice on to the basalia, thus extending the area of the fin. Radialia of bone may be regarded as deriva- tives of the bony scales of fishes. Frequently isolated basalia, usually situated directly in front of the fin proper, develop into defensive spines that are augmented by basal poison sacs. In addition there is present in many fishes still a third kind of fin rays {actinotrichia), which are horny in texture, dermal in origin, and double in structure (Fig. 548), one half being derived from either side of the fold of skin constit
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