The heterocercal tail is one in which the hindmost vertebræ are bent upwards. The term is generally applied to those fishes only in which this bending is considerable and is externally evident, as in the sharks and Ganoids. The character disappears by degrees, changing sometimes to diphycercal or leptocercal by a process of degeneration, or in ordinary fishes becoming homocercal. Dr. Ryder uses the term heterocercal for all cases in which any up-bending of the axis takes place, even though it involves the modification of but a single vertebra. With this definition, the tail of salmon, herring, and even of most bony fishes would be considered heterocercal, and most or all of these pass through a heterocercal stage in the course of development. The term is, however, usually restricted to those forms in which the curving of the axis is evident without dissection.
Fig. 66.—Coryphænoides carapinus (Goode and Bean), showing leptocercal tail. Gulf Stream.
The homocercal tail is the fan-shaped or symmetrical tail common among the Teleosts, or bony fishes. In its process of development the individual tail is first archicercal, then lophocercal, then diphycercal, then heterocercal, and lastly homocercal. A similar order is indicated by the sequence of fossil fishes in the rocks, although some forms of diphycercal tail may be produced by degeneration of the heterocercal tail, as suggested by Dr. Dollo and Dr. Boulenger, who divide diphycercal tails into primitive and secondary.
The peculiar tapering tail of the cod, the vertebræ growing progressively smaller behind, is termed isocercal by Professor Cope. This form differs little from diphycercal, except in its supposed derivation from the homocercal type. A similar form is seen in eels.
Fig. 67.—Heterocercal tail of Young Trout, Salmo fario (Linnæus). (After Parker and Haswell.)
The term leptocercal has been suggested by Gaudry, 1883, for those tails in which the vertebral column ends in a point. We may, perhaps, use it for all such as are attenuate, ending in a long point or whip, as in the Macrouridæ, or grenadiers, the sting-rays, and in various degenerate members of almost every large group.
The term gephyrocercal is devised by Ryder for fishes in which the end of the vertebral axis is aborted in the adult, leaving the caudal elements to be inserted on the end of this axis, thus bridging over the interval between the vertical fins, as the name (γεφύρος, bridge; κέρκος, tail) is intended to indicate. Such a tail has been recognized in four genera only, Mola, Ranzania, Fierasfer, and Echiodon, the head-fishes and the pearl-fishes.