The Skeleton of the Fish 



55 



which the bony framework has attained completion. But to 

 understand the origin and relation of parts we must begin with 

 the lowest types, tracing the different stages 

 in the development of each part of the 

 system. 



In the lancelets (Leptocardii) , the verte- 

 bral column consists simply of a gelatinous 

 notochord extending from one end of the 

 fish to the other, and pointed at both ends, 

 no skull being developed. The notochord 

 never shows traces of segmentation, although 

 cartilaginous rods above it are thought to 

 forecast apophyses. In these forms there is 

 no trace of jaws, limbs, or ribs. 



In the embryo of the bony fish a similar 

 notochord precedes the segmentation and 

 ossification of the vertebral column. In 



M 



FIG. 40. Maxillary and 

 premaxillary of Sebas- 

 tolobus alascanus. M, 

 maxillary; PM, pre- 

 maxillary. 



most of the extinct types of fishes a notochord more or less 



FIG. 41. Part of skeleton of Selene vomer (Linnaeus). 



