CLASS CYCLOSTOMATA 



343 



amphioxus, and undergoes a metamorphosis when it becomes adult. 

 In the adult the endostyle becomes the thyroid gland. 



362. Relationship of the Cyclostomes. — The characteristics which 

 have been given for these animals show distinctly adaptation to a parasitic 

 mode of life. However, the relatively large number of gill slits, the pres- 

 ence of a pronephros in the adult hagfish, and the condition of the brain 



P/necr/ 

 body 



A B 



Fig. 239. — Brain of lamprey. A, dorsal view; B, lateral view. (From a Ziegler model, 

 after Wiedersheim.) The roots of the cranial nerves are marked by roman numerals. 



all point to the fact that the cyclostomes are more primitive than other 

 vertebrates. Some of the amphioxus-like characteristics of the larval 

 lamprey seem to show the inheritance of characteristics possessed by a 

 common ancestor of both cephalochordates and vertebrates. 



363. Economic Importance. — The hagfish has never been an article 

 of food, but the flesh of the lamprey is sometimes eaten both in Europe 

 and in America. Both of these animals, however, are to be considered 

 as economically injurious. 



