112 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



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 connexion with either of 

 the others. 



Fig. 1 17. — Part of skeleton of dorsal fin of perch, b, basalia; r, radialia; s, soft fin rays. 



The simplest skeleton (fig, 117) consists of a meta meri c series^ 

 c artila ge orosseous bars, each^ usually divided into a deeper basale 

 and a more distal_radiale, the former frequently articulating with 

 or alternating with the spinous processes of the vertebrae, while the 

 radialia 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 epi- 

 dermis. Frequently they are united into bundles (soft fiin rays, s) 

 and they may replace the radiaha. 



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 formation. Apparently the simplest fin known is that of the extinct 

 shark, Cladoselache (fig. 118), 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 basalia, the basalia of the two sides being unconnected 

 with each other. Greater growth of the basalia would result in some of them 

 meeting and fusing in the middle Une, thus forming a bar across the ventral side 



