CLASS CYCLOSTOMATA 



337 



amphioxus, and undergoes a metamorphosis when it becomes adult. 

 In the adult the endostyle becomes the thyroid gland. 



360. 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 

 presence of a functional pronephros in the adult hagfish, and the condition 



Pineal 

 body 



Olfactory,^ 

 y lobe 



Cerebrum 

 Diencephalon 



Optic lobes 



^ W 



Cerebellum ^ 



"^ Yn 



VTTT 



Medulla 



A B 



Fig. 226. — 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. 



of the brain 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. 

 361. 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. 



