lyS 



CHORDATE ANATOMY 



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 

 between the dorsal extensor muscles of the appendage and the ventral 

 flexor muscles. 



m'////my//////////y^^ 



MEDIAN FIN-FOLD 



PECTORAL FIN ^^,.T PELVIC FIN 

 ANUS 



PRIMITIVE RADIALS 



PELVIC GIRDLf 



Fig. 167. — 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-E 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 than the left. In E the 

 skeletons of the girdle and extremity are differentiated. (After Wiedersheim.) 



Further steps in the evolution of the appendicular skeleton involve 

 the thickening and fusion of the basal or proximal portions of the radial 

 cartilages and the extension of the basal cartilages thus formed into the 

 body-wall and towards the mid-ventral hne. The result of this appears 

 today in the pelvic fin of elasmobranchs. The beginnings of a girdle are 

 seen in a ventral cartilaginous plate, the ischio-pubis. A doubtful begin- 

 ning of the ilium may be seen in the so-called iliac process. Evolution in 



