APPENDICULAR SKELETON 



PAIRED APPENDAGES 



227 



The limbless Cyclostomes excepted, most members of every 

 class of Vertebrates have typically two pairs of appendages (fig. 232), 

 an anterior (pectoral) pair, arising primitively just behind the last 

 gill-cleft; and a posterior (pelvic or ventral) pair just in front of the 

 anus. A few genera of all classes may lack one or both pairs, while 

 the position may vary in members of the different groups. 



The most primitive condition of the paired appendages occurs in 

 fishes, those of Tetrapoda being very different. Fishes have paired 

 fins (pterygia), paddle-like appendages; Tetrapoda have jointed legs 

 (podia). Pterygia are the more primitive, and from them podia 

 have undoubtedly arisen, although the steps in this evolution are 

 far from evident. Both arise, ontogenetically, as longitudinal folds 



my 



m 



Fig. 240. 



-Budding of appendicular muscles from myotomes in Pristiurus (Rabl). 

 b, muscle buds; my, myotomes. 



extending over several somites in the lateral trunk region (fig. 240), 

 the number of somites concerned frequently being greater than that 

 entering the definitive appendage, the extreme number being reached 

 in the pectoral fin of skates. Each fold is a dupHcature of ectoderm, 

 carried out by increase in the underlying mesoderm, the fold con- 

 taining muscle-buds, nerves, blood vessels and scleroblasts. 



At first the fin folds are horizontal with dorsal and ventral sur- 

 faces, anterior (preaxial) and posterior (postaxial) borders, relations 

 persisting in the pectoral fins of adult skates. Elsewhere the 

 anterior border of each fin grows faster than the posterior so that the 

 fin becomes triangular or trapezoidal. Also, in most fishes, and 

 easiest recognized in the pectorals, the Hne of attachment of fin to 

 trunk shifts so that the anterior border is moved upwards and the 

 original ventral surface is turned towards the front. This may go so 

 far with the pectoral fin that the fine of attachment to the body 

 becomes vertical and the dorsal side of the fin is turned towards the 



