. Comparative anatomy. Anatomy, Comparative. PECTORAL FIN PRIMITIVE RADIALS T PELVIC FIN ANUS PELVIC GIRDLf. Fig. 193.—Diagram illustrating the hypothetical evolution of the paired fins and their skeletal supports. A represents the primitive stage of continuous fin-folds. The dorsal fin and the ventral fin posterior to the anus are median and unpaired. B is the definitive Elasmobranch stage. The paired fin folds persist only in the region of pectoral and pelvic fins. The median fins also become discontinuous. C~R illustrate hypo- thetical stages in the evolution of the skeleton of the pelvic f
. Comparative anatomy. Anatomy, Comparative. PECTORAL FIN PRIMITIVE RADIALS T PELVIC FIN ANUS PELVIC GIRDLf. Fig. 193.—Diagram illustrating the hypothetical evolution of the paired fins and their skeletal supports. A represents the primitive stage of continuous fin-folds. The dorsal fin and the ventral fin posterior to the anus are median and unpaired. B is the definitive Elasmobranch stage. The paired fin folds persist only in the region of pectoral and pelvic fins. The median fins also become discontinuous. C~R illustrate hypo- thetical stages in the evolution of the skeleton of the pelvic fins of Elasmobranchs. The right side of C and E represents a later stage in phylogenesis. In £ the skeletons of the girdle and extremity are differentiated. (After Wiedersheim.) ontogenesis. In elasmobranchs the muscle buds which invade the fin- folds are metamerically arranged. The third step in this evolution was the appearance of a series of inter- myotomic cartilage rays like those which support both median and paired fins of elasmobranchs. The universal occurrence of skeletal material in connexion with muscle, and indeed wherever in an organism stresses occur, may possibly be taken as explaining these radial cartilages. The position between the myotomes is obviously adaptive, as is also their position. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : P. Blakiston's Son
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