Motor System — Muscles and Skeleton 



MEDIAN FIN-FOLD 



135 



PAfRED FIN-FOLDS 



MEDIAN FIN 



ANUS 



PECTORAL FIN 



PELVIC FIN 



Fig. 127. Diagram illustrating the hypothetic evolution of the paired fins. (A) 

 The primitive stage of continuous fin-folds. The dorsal fin and the ventral fin 

 posterior to the anus are median and unpaired. (B) The definitive elasmobranch 

 stage. The paired fin-folds persist only in the region of pectoral and pelvic fins. 

 The median fins also become discontinuous. (After Wiedersheim. Courtesy, Neal 

 and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



'-d 



Fig. 128. Esox lucius, L. (A) 

 Two radials of the dorsal fin, 

 left side view. (B) Radial and 

 dermal ray seen from in front. 

 (d.) Distal cartilage; (/.) dermal 

 lepidotrich, broken short in (A) ; 

 (m.) median segment and (pr.) 

 proximal segment of radial. 

 (From Goodrich: "Studies on 

 the Structure and Development 

 of Vertebrates." By permission 

 of The Macmillan Company, 

 publishers.) 



