CHAPTER IV 

 THE SKELETON OF THE FISH 



PECIALIZATION of the Skeleton. — In the lowest form 

 of fish-like vertebrates (BrancJiiostoma), the skeleton 

 consists merely of a cartilaginous rod or notochord 

 extending through the body just below the spinal 

 cord. In the lampreys, sharks, dipnoans, crossopterygians, 

 and sturgeons the skeleton is still cartilaginous, but grows 

 progressively more complex in their forms and relations. 

 Among the typical fishes the skeleton becomes ossified and 

 reaches a very high degree of complexity. \"ery great varia- 

 tions in the forms and relations of the difterent parts of the 

 skeleton are found among the bony fishes, or teleostei. The 

 high degree of specialization of these parts gives to the study 

 of the bones great importance in the systematic arrangement 

 of these fishes. In fact the true affinities of forms is better 

 shown by the bcmes than by any other system of organs. In a 

 general way the skeleton of the fish is homologous with that of 

 man. The head in the one corresponds to the head in the other, 

 the back-bone to the back-bone, and the paired fins, pectoral 

 and ventral, to the arms and legs. 



Homologies of Bones of Fishes. — But this homology does 

 not extend to the details of structure. The bones of the arm 

 of the specialized fish are not by any means identical with the 

 humerus, coracoid, clavicle, radius, ulna, and carpus of the higher 

 vertebrates. The A-ertcbrate arm is not derived from the 

 pectoral fin, but both from a cartilaginous shoulder-girdle with 

 undifferentiated pectoral elements bearing fin-rays, in its details 

 unlike an arm and unlike the pectoral fin of the speciahzed fish. 

 The assumption that each element in the shoulder-girdle and 

 the pectoral fin of the fish must correspond in detail to the 

 arm of man has led to great confusion in naming the difterent 



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