PISCES 159 



and the male remains in the burrow guarding the eggs till they hatch 

 out into tadpole-like larvae. While guarding the eggs the pelvic 

 fins of the male act as accessory external gills, the fins being cov- 

 ered with numerous vascular filaments, *^** 



GENERALIZED AND SPECIALIZED TYPES OF FISHES AND THE 

 AXIAL GRADIENT THEORY OF STRUCTURAL RELATIONS 



In nearly all of the orders or sub-orders of fishes there are species 

 that have retained a well-balanced relation of head, body, and tail; 

 that have the moderately elongated, cylindrical, double-pointed 

 shape; with smooth body, lacking heavy armature; without exaggera- 

 tions of the fin-system ; and with rather dull coloration. The dog-fishes 

 among elasmobranchs, Polypterus among the crossopterygians, her- 

 ring, pike, trout, killifish, cod, perch, mackerel, etc., among the 

 teleosts; all these have, to a more or less complete extent, retained the 

 generalized characters of the ancestral prototype of all the fishes. 

 They all agree quite closely with the very ancient types that have 

 come down to us in fossil form. All of these generalized types are 

 active, predaceous fishes, with an abundant supply of energy; they 

 are youthful in the physiological sense. 



Specialization in fishes, as in other groups of vertebrates, follows 

 certain definite lines and results in several types of structural modifi- 

 cation. One of the commonest of these is the eel-like type, which 

 appears in all of the dominant sub-orders. In fishes of this type the 

 body is greatly elongated, and the trunk and tail are apt to be propor- 

 tionately more highly developed than the head. The head remains 

 small (microcephalic) and exhibits a number of degenerate features, 

 such as small eyes and imperfect branchial openings. The paired 

 fins are usually absent and the median fins are of the primitive, un- 

 differentiated diphy cereal type. Perhaps the most conspicuous 

 example of reduced head is seen in the Hawaiian eel, Callechelyx luteus, 

 a species in which the diameter of the body is about twice that of the 

 head. Modern types of eel-like fishes are usually scaleless, but some 

 of the archaic forms, such as the crossopterygian species, Calamichthys, 

 are heavily scaled. In general, the eel-like type may be interpreted 

 as a result of a suppression of the head parts and a consequent relative 

 increase in the development of the body and the tail. 

 ' The antithesis of the eel-like type is that in which the head parts 

 are abnormally large (megacephalic) and the body and tail relatively 



