SKELETON. 103 



dorsally until the two met and fused in the middle line of the back, thus producing 

 a continuous dorsal fin. The ventral folds migrated downward in the same way, 

 eventually meeting behind the vent, but that opening 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. 65) 

 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. 

 Primitively, and in many species through life, they are continuous, but 

 usually gaps occur during development so that the fins of the adult are 

 separated by intervals from each other. They occur in practically all 

 fishes, in larval and tailed amphibians, and in isolated groups like the 

 ichthyosaurs and whales. In amphibians and higher groups the 

 median fins have no skeleton, but elsewhere it is of cartilage, bone, 

 or a horny substance (elastoidin) , the latter being the most constant 

 and occurring in connection with either of the others. 



The simplest skeleton consists of a metameric series of cartilage or 

 osseous bars, each usually divided into a deeper basale and a more 

 distal radiale, the former frequently articulating or alternating with 

 the spinous processes of the vertebrae, while the latter support the fin 

 proper. The elastoidin elements consist of a number of slender rods 

 (actinotrichia), outnumbering the somites, and arising from the 

 corium, immediately below the epidermis. Frequently they are 

 united into bundles (soft fin rays) and may replace the radialia. 



PAIRED APPENDAGES. 



The paired appendages are not, as the gill-arch theory would demand, derived 

 from a single somite, but a varying number of segments participate in their forma- 

 tion. Apparently the simplest fin known is that of the extinct shark, Cladoselache 

 (fig. 107), in which it is a rounded lobe supported by a number of rods, like the 

 radialia in a median fin. These are attached proximally to a few larger plates, the 



