A treatise on zoology . igs. 63,64). The homocercal caudalfin accpiires a secondary oiit-ward sjmmetry. As Huxley[226] and Agassiz [2] long agoshowed, with the morespecialised tails pass throughthe more primitive stages indevelopment: from the di-phycercal to the heterocercal,and thence to the homocercaltype (Fig. 65). But in theprocess the middle or axiallobe, in which runs the noto-chord, becomes relatively re-duced ; so that, whereas inprimitive diphycercal andheterocercal tails the noto-chord reaches to or near thetip (Elasmobranchs, Dipnoi, Chondrostei), in the more abbreviatehetero
A treatise on zoology . igs. 63,64). The homocercal caudalfin accpiires a secondary oiit-ward sjmmetry. As Huxley[226] and Agassiz [2] long agoshowed, with the morespecialised tails pass throughthe more primitive stages indevelopment: from the di-phycercal to the heterocercal,and thence to the homocercaltype (Fig. 65). But in theprocess the middle or axiallobe, in which runs the noto-chord, becomes relatively re-duced ; so that, whereas inprimitive diphycercal andheterocercal tails the noto-chord reaches to or near thetip (Elasmobranchs, Dipnoi, Chondrostei), in the more abbreviateheterocercal and homocercal tails of the Holostei the hypochordalfin projects far beyond it (Figs. 61-66). As will appear later, it seems probable that the heterocercal tailhas been independently acquired in more than one group (p. 233);and that, in some cases, fish whose ancestors possessed fins have reacquired an outward symmetry, making themsuperficially similar in appearance to the primitive diphycerca. Fic. CO. Successive stages in the development of tliehomocercal tail of the Flounder, Plearonectes flesiis,L., siiowing the disappearance of the axial lobe, c,and growth of the hypochordal fin, ac, actino-trichia ; , liaemal arch ; hy, hypural cartilage ; /,dermal ray; , neural spine ; nt, notocliord.(After A. Agassiz.) MEDIAN FINS 105 type (p. 480). The name gephyrocorcal has been ai)plied to thesepseiido-diphycercal tails. Turniii--- now to the relation between the axial and the appen-dicular skefeton of the median fins, which has already been alludedto above(pp. 69 and 101), wefind that in the Pleuracanthids amongstthe Chondrichthyes, in the Dipnoi, and in many Teleostomes (Coela-canths, many Teleosts) the radials of the fins correspond to, andmay articulate with, the neural and haemal spines. As a rule, themore complete is the fin, the more thorough is the contniuity d/f.
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