Some points in the function, development and evolution of the tail in fishes

Authors


  • *Thesis approved for the degree of M. Sc. in the University of London.

SUMMARY.

  • 1 Models show that in fishes the effective action of a tail in a vertical direction depend on:—
  • (a) the relative size of the epichordal and hypochordal lobes;
  • (b) the flexibility of the fins;
  • (c) the direction of the terminal part of the axis;
  • (d) the relative sizes of the dorsal and ventral lobes of a homocercal tail.
  • 2 The tails of Carassius auratus, Macropodus opercularis and Betta splendens pass through stages which may be called protocercal and heterocercal before the final homocercal tail is formed.
  • 3 Because of the precocity of the appearance and functioning of the swimbladder, which causes the specific gravity of the fish to become equal to that of the water in which it lives, no vertical reaction is necessary to keep the larval fish off the bottom when it is swimming. The tail, therefore, is isobatic throughout development.
  • 4 Whilst passing through the heterocercal stage the flexible epichordal lobe is retained to counteract the upward reaction on the large hypochordal lobe. In ontogeny, therefore, the function of the heterocercal stage of a homocercal tail differs from that of the heterocercal tail found in adult fishes.
  • 5 In spite of retaining the epichordal lobe during the heterocercal stage, the development of the homocercal tail is primarily recapitulatory.
  • 6 The possible evolution of tails from a primitive type is discussed in relation to their function and to the specific gravity of the fishes.
  • 7 It is suggested that the protocercal tail was the primitive type from which were evolved the heterocercal type (associated with pectoral fins) and the hypocercal type (associated with the absence of pectoral fins). The evolution of these tails would enable animals to swim although their specific gravity had increased.
  • 8 From these two types, five evolutionary lines are traced, each of which terminates in a tail which is exactly, or almost exactly, symmetrical.
  • 9 The achievement of external symmetry usually signifies that the tail takes no part in raising the posterior part of the animal when it is swimming, or that it is degenerate or specialized.

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