The form of the tail at the base of the caudal fin varies in the different groups. In most primitive types, as in most embryonic fishes, the vertebræ grow smaller to the last (diphycercal). In others, also primitive, the end of the tail is directed upward, and the most of the caudal fin is below it. Such a tail is seen in most sharks, in the sturgeon, garpike, bowfin, and in the Ganoid fishes. It is known as heterocercal, and finally in ordinary fishes the tail becomes homocercal or fan-shaped, although usually some trace of the heterocercal condition is traceable, gradually growing less with the process of development.
Since Professor Agassiz first recognized, in 1833, the distinction between the heterocercal and homocercal tail, this matter has been the subject of elaborate investigation and a number of additional terms have been proposed, some of which are in common use.
A detailed discussion of these is found in a paper by Dr. John A. Ryder "On the Origin of Heterocercy" in the Report of the U. S. Fish Commissioner for 1884. In this paper a dynamic or mechanical theory of the causes of change of form is set forth, parts of this having a hypothetical and somewhat uncertain basis.
Dr. Ryder proposes the name archicercal to denote the cylindroidal worm-like caudal end of the larva of fishes and amphibians before they acquire median fin-folds. The term lophocercal is proposed by Ryder for the form of caudal fin which consists of a rayless fold of skin continuous with the skin of the tail, the inner surfaces of this fold being more or less nearly in contact. To the same type of tail Dr. Jeffries Wyman in 1864 gave the name protocercal. This name was used for the tail of the larval ray when it acquires median fin-folds. The term implies, what cannot be far from true, that this form of tail is the first in the stages of evolution of the caudal fin.
To the same type of tail Mr. Alexander Agassiz gave, in 1877, the name of leptocardial, on the supposition that it represented the adult condition of the lancelet. In this creature, however, rudimentary basal rays are present, a condition differing from that of the early embryos.
The diphycercal tail, as usually understood, is one in which the end of the vertebral column bears "not only hypural but also epural intermediary pieces which support rays." The term is used for the primitive type of tail in which the vertebræ, lying horizontally, grow progressively smaller, as in Neoceratodus, Protopterus, and other Dipnoans and Crossopterygians. The term was first applied by McCoy to the tails of the Dipnoan genera Diplopterus and Gyroptychius, and for tails of this type it should be reserved.
Fig. 64.—Heterocercal tail of Bowfin, Amia calva (Linnæus). (After Zittel.)
Fig. 65.—Heterocercal tail of Garpike, Lepisosteus osseus (Linnæus).