42 



STRUCTURE OF THE VERTEBRATES 



Order Teleostei form the largest group of the fish, and express 

 the characteristics which one thinks of as "fishy". They include 

 all the common food and game fish, from the more primitive 

 Tuna to the highly specialized globe-fish and the sea horse. 

 Though they lie entirely out of the line of evolution of the higher 



■ Notochord 



Fig. 11. Tail Skeleton of the Stur- 

 geon. The notochord extends almost 

 to the tip of the tail and the radial 

 cartilages are practically equal on 

 dorsal and ventral sides. A typical 

 heterocercal tail. 



Notochord 



Fig. 12. Tail Skeleton of the Gar- 

 pike. The notochord has shortened, 

 and the ventral radials have elon- 

 gated, giving a homocercal appear- 

 ance. 



Fig. 13. Tail Skeleton of a Teleost 

 Fish. The notochord has been oblit- 

 erated and the caudal bones are al- 

 most homocercal. Other teleost fishes 

 carry the modification further. 



groups, there is no more perfect order for a study of adaptive 

 radiations and specializations. Technically they are divided into 

 thirty or more orders, and numerous sub-orders; and there are 

 more than two hundred families. 



The scales typically overlap and have lost the heavy bony 

 structure which is found in the more primitive forms. The tail 



