The Skeleton of the Fish 



35 



bones. Among the many bones of the fish's shoulder-girdle 

 and pectoral fin, three or four different ones have successively 

 borne the names of scapula, clavicle, coracoid, humerus, radius, 

 and ulna. None of these terms, unless it be clavicle, ought by 

 rights apply to the fish, for no bone of the fish is a true homo- 

 logue of any of these as seen in man. The land vertebrates and 

 the fishes have doubtless sprung from a common stock, but this 

 stock, related to the crossopterygians of the present day, was 

 unspecialized in the details of its skeleton, and from it the fishes 

 and the higher vertebrates have developed the widely diverging 

 lines. 



Parts of the Skeleton. — The skeleton may be divided into 

 the head, the vertebral column, and the limbs. The very lowest 

 of the fish-like forms {BrancJiiostoma) has no differentiated head 



Fig. 21. — Striped Bass, Roccu.t linealiis (Bloch). Potomac Ri^-er. 



or skull, but in all the other forms the anterior part of the 

 vertebral column is modified to form a cranium for the protec- 

 tion of the brain. In the lampreys there are no jaws or other 

 appendages to the cranium. 



In the sharks, dipnoans, crossopterygians, ganoids, and teleosts 

 or bony fishes, jaws are developed as well as a variety of other 

 bones around the mouth and throat. The jaw-bearing forms 

 are sometimes known by the general name of gnathostomes. 

 In the sharks and their relatives (rays, chimteras, etc.) all the 

 skeleton is composed of cartilage. In the more speciaHzed 

 bony fishes, besides these bones we find also series of mem- 

 brane bones, more or less external to the skull and composed of 



