AN INTRODUCTION TO ZOOLOGY. 



described under the name of Ammocoetes ; it is an 



interesting excep- 



tion to the general 

 rule that Verte- 

 brates, excepting 

 Amphibia, do not 

 have larval forms. 



The Sharks, 

 Skates, and Dog- 

 fishes, or Selachii 

 {Elasmohr anchs), 

 are also called Chon- 

 dropterygii, or 

 gristle-finned ; the 

 skeleton is for the 

 most part made up 

 of cartilages in- 

 stead of bones. 

 They have, like 

 most other Verte- 

 brates, four limbs, 

 two pectoral fins 

 and two pelvic fins ; 

 and, in addition to 

 these, they have 

 median fins. In 

 some important re- 

 spects, these fishes 

 are very highly or- 

 ganized ; they have 



