THE STRUCTURE OF FISHES 61 



man's thumb. Although the fish brain has most of the parts found in 

 the brains of land vertebrates, many parts are not fully developed. 

 Fewer cranial nerves are present. The forepart, or cerebrum, which 

 forms the thinking region of the human brain, is undeveloped in the fish. 

 The skeleton is relatively complex. The skull when fully developed 

 contains an enormous number of bones. Many of the bones found in 

 fishes are present in land animals, but in land animals the number is 

 greatly reduced by the fusion of many bones to form single bones. Fishes 

 often have an extra rib, peculiar to them, which enables them to have 

 an inner and outer pair of ribs for each trunk vertebra. Together with 

 the accessory bones between the muscles these ribs make some species 

 exceedingly bony. 



