SKELETON 



III 



radialia which support the gills, one radial forming an axis, the adjacent radials 

 being arranged on either side of this, and carried outward from the arch by the 

 growth of the septum to form the body of the appendage (fig. 133). A some- 

 what similar view is that the appendage itself is a modification of an external 

 gill, such as is found in larval amphibians. 



Another view supposes an ancestor with two pairs of longitudinal folds 

 running the length of the body behind the head, each fold supported by a 

 series of skeletal rods (fig. 116). With farther development the upper folds 

 on either side migrated dorsally until the two met and fused in the middle line 

 of the back, thus producing a continuous dorsal fin. The ventral folds migrated 



^4!^^ 



Fig. 116. — Diagram of the origin of median and paired appendages from lateral 



fin folds. 



downward in the same way, eventually uniting behind the vent, but that open- 

 ing prevented their meeting farther forward. From the fused part behind the 

 vent the anal and the lower part of the caudal fins were formed, while the 

 paired appendages are differentiations of the preanal parts of the ventral 

 longitudinal folds. 



It may be said that in development there is no such double origin of the 

 dorsal fin. In several sharks the paired fins arise from continuous folds, while 

 in the Japanese gold fish the anal fins are frequently paired and the caudal has 

 a double condition below, such as would result from the failure of folds to unite 

 in this region. In criticism of the gill-arch theory it may be said that the 

 supports of the paired appendages arise outside of the body musculature, while 

 the visceral arches (p. 73) are internal. 



The Median Appendages 



The median or azygos appendages always have the form of fins, 

 and may be dorsal, terminal (caudal) or ventral (anal) in position. 



