THE ENDOSKELETON 



699 



Both theories are unsatisfactory, the latter because there is no double 

 origin of the dorsal fins, and the former, known as the gill-arch-theory, 

 is unsatisfactory due to the fact that the paired appendages develop out- 

 side the body musculature, while the visceral arches are always internal. 



All median appendages have the form of fins, and are termed dorsal, 

 terminal (caudal), or ventral (anal). They may occur as a continuous 

 fin, or they may be broken up with intervals between. Fins occur in all 

 fishes, in the larval and tailed amphibians, and even in rather isolated 

 groups, such as the Ichthyosaurs and the whales, although in the am- 

 phibians and the higher groups, there is no skeleton in the median fins. 



The skeleton of the fins usually consists of definite metameric car- 

 tilaginous or osseous bars, each of which is divided into a proximal 

 basale and a distal radiale. The basale often articulates or alternates 

 with the spinous processes of the vertebrae. The radiale supports the 

 fins proper. 



In the higher forms of vertebrates, when the skeleton of the fins is 

 not composed of cartilage or bone, there is a horny substance known 

 as elastoidin which forms of a number of slender rods in greater number 

 than the somites. They arise from the corium just below the epidermis, 

 often being united in bundles, and thus form soft-finned rays, often re- 

 placing the radiale. 



PAIRED APPENDAGES 



An extinct shark had a pair of fins approximately in the region where 

 pectoral and pelvic girdles normally form, but no satisfactory theory has 

 yet developed as to how arms and legs were derived from this type of fin. 



