cci STRUCTURE OF VERTEBRATES 



suggesting a heterocercal origin, as the notochordal axis is slightly 

 upturned and much shortened, so that the fin is supported almost 

 entirely by the rays. This condition is typical of the teleosts, and 

 its development through the heterocercal state can be followed 

 in embryology (Fig. 431). 



When the fishes left the water and became tetrapods the un- 

 paired fins were lost. Substitutes have, however, been acquired 

 by nearly all the vertebrates which have returned to a fully 

 aquatic existence ; these include the Ichthyosauria amongst 

 reptiles and the whales amongst mammals. In the whales the 

 caudal fin is flattened horizontally instead of vertically, but 

 cinema photographs have shown that, except for this difference, 

 the method of swimming is similar to that of the fish. In none 

 of these secondarily acquired unpaired fins are fin-rays present. 



PAIRED LIMBS 



The structure of the paired fins of fish is generally similar to 

 that of the unpaired fins ; there are proximal bony skeletal 

 elements and distal rays, both surfaces are covered wdth muscle, 

 and the whole is covered with skin. This similarity suggests a 

 similar origin, and it is reasonable to suggest that just as the 

 unpaired fins other than the caudal developed as devices to prevent 

 rolling, so the lateral fins developed to prevent pitching. To make 

 things appear neater it has further been suggested that originally 

 the lateral fins, like the dorsal and ventral ones, ran most or all 

 the length of the body. This ' lateral fin fold theory ' has a small 

 amount of fossil support in that the early shark Cladoselache 

 and the agnathan Cephalaspis appear to have had fins which 

 merge gradually into the body fore and aft, and so look like 

 restricted portions of a continuous fold. Some early sharks have 

 more than two pairs of fins ; Climatius, for example, has seven 

 (Fig. 432). 



The details of the fin skeleton of early fishes vary rather widely, 

 and it is difficult to take any one arrangement as an ancestral 

 or central type. The student will not be greatly misled if he takes 

 the familiar arrangement of the dogfish pectoral fin (Fig. 241) 

 as a starting-point ; there are three large basals, several radials 

 distal to this, then polygonal plates, and finally the horny rays 

 supporting the skinny part of the fin. The pelvic fin is a variant 

 of this with a single basal, and other sharks have other numbers. 



