ISSN:0368-2935    

DOI:10.1111/j.1096-3642.1896.tb00395.x

出版商:Blackwell Publishing Ltd    

年代:1896

数据来源: WILEY

ISSN:0368-2935    

DOI:10.1111/j.1096-3642.1896.tb00396.x

出版商:Blackwell Publishing Ltd    

年代:1896

数据来源: WILEY

ISSN:0368-2935    

DOI:10.1111/j.1096-3642.1896.tb00398.x

出版商:Blackwell Publishing Ltd    

年代:1896

数据来源: WILEY

ISSN:0368-2935    

DOI:10.1111/j.1096-3642.1896.tb00399.x

出版商:Blackwell Publishing Ltd    

年代:1896

数据来源: WILEY

摘要:

SUMMARYIn this section it is proposed to institute a comparison of the principal modifications of the radial elements of the mesial fins with regard to their degree of segmentation, the extent to which they are affected hy degeneration and concrescence, and the variable modes of support they offer to the fin‐rays, in different groups of Fishes.The most primitive type of radial element is to he found in the Marsipobranchs, where they exist in the form of unsegmented cartilaginous rods, either simple or dichotomously branched towards their distal ends, and, in the absence of horny fibres or fin‐rays, they extend to the peripheral margins of the fins and constitute their sole skeletal support.In retaining the condition of simple unsegmented cartilaginous rods, the radial elements of the Holocephala resemble those of the Marsipobranchs; but how far the simplicity of these structures is primitive, or has been acquired by the suppression of segments, cannot at present be determined. Actinotrichia in the form of norny fibres support the periphery of the fins.In the most primitive of extinct Elasmobranchs (e. g.Clado‐selache, Pleuracanthus)the radial elements of the dorsal fins become complicated by segmentation, each being divided into a basal and a distal segment, of which the distal is the longer. As pointed out by Dean [11], the various elements extend to the periphery of the fin and in conjunction with horny fibres, which inCladoselacheare of secondary importance and lie between the former, contribute to the support of the fin.In the Arthrodira (e. g.Coecosteus)[Smith Woodward, 12] the radial elements are very similar bisegmental structures.In existing Elasmobranchs the typically rod‐like cartilaginous radial elements are generally trisegmental, exhibiting a division into proximal, mesial, and distal segments, flexibly connected with one another by ligament, and in fairly close apposition throughout their length for mutual support. The central or approximately central elements are usually the longest, and almost invariably the most anterior and posterior undergo reduction in length and lose one or more of their constituent segments–facts which find their legitimate explanation in the partial atrophy of an originally more extensive fin and the concentration of the persistent residue of the fin‐supports. The horny fibres, as was probably also the case in the fossil Elasmobranchs above‐mentioned, are much more numerous than the supporting cartilages, and to a greater extent than in extinct types they supplant the latter in supporting the flexible peripheral margins of the fins. As has already been pointed out, the radial elements are liable to considerable modifications in different genera through(a)the longitudinal concrescence of the proximal segments, or of both proximal and mesial;(b)the suppression of particular segments in certain of the elements; and (c) the apparently secondary subdivision of the distal segments.The polymorphic character of existing Ganoids is well illustrated by the existence of striking variations in the structure of the radial elements, of which three well‐marked types are represented within the limi

ISSN:0368-2935    

DOI:10.1111/j.1096-3642.1896.tb00400.x

出版商:Blackwell Publishing Ltd    

年代:1896

数据来源: WILEY

ISSN:0368-2935    

DOI:10.1111/j.1096-3642.1896.tb03434.x

出版商:Blackwell Publishing Ltd    

年代:1896

数据来源: WILEY