File:A treatise on zoology (1900) (14783345445).jpg

Identifier: treatiseonzoolo09lank (find matches)
Title: A treatise on zoology
Year: 1900 (1900s)
Authors: Lankester, E. Ray (Edwin Ray), Sir, 1847-1929 Bather, Francis Arthur, 1863-1934 Goodrich, Edwin S. (Edwin Stephen), 1868-1946
Subjects: Zoology
Publisher: London : A. & C. Black
Contributing Library: Robarts - University of Toronto
Digitizing Sponsor: University of Toronto

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47). The exact correspondence between the body segments and thefin segments is usually disturbed by concentration and byconcrescence, which accompany the breaking up of the continuousfold into separate fins. The base of each separate portion tends tobecome narrower, and thus a dorsal fin which in the embryOdeveloped from some dozen segments, may in the adult come tooccupy only some half-dozen or fewer. As a rule, more buds arisein the emWyo than come to full development in the adult, somebeing suppressed at each end, where concentration is most pro-nounced. The skeletal and muscular elements thus becomerelatively compressed, and this concentration may take place moreat one end of the fin than at the other (Fig. 47). Owing toconcentration radial fin-muscles may become more or less widelyseparated from the myotomes which gave rise to them, and thenerves are made to converge towards the narrowed base of the finto supply them; thus are formed longitudinal collector nerves, 72 PAIRED FINS
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2s3g^ c ^ ^ m as . o S t S 9 S ■* ^ otE = ^3 2= -r.^ ^ ■- 5 IT t^ •- .E s ifSE cs ./ tc ~ •a oSS ^ - =: c :».!: — ^ c -•J-. = c — e a -/: 5 ■:: ^ ¥ bS PAIRED FINS 73 especially in front of the fins. Concrescence, the second factor,may introduce profound modification (Thacher (434), Mivart (300)).Not only do the skeletal radials often fuse, forming basal plates orjointed axes of most varied shapes (Figs. 48-50, 52-54), but themuscular segments also possibly mix to some extent in ontogeny,and lose their primitive metamerism. The very important subject of the origin of the paired limbs ofGnathostomes must now be discussed. As it is generally agreedthat the primitive form of these must have been the fin-like ichthyopterygium, the evolution of the fish fin only need for thepresent be considered. What appear to be the less modified fishpossess a pair of i)ectoral fins supported by a pectoral girdle behindthe gills, and a pair of pelvic fins supported by a pelvic girdlepass

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