I < -1 1 1 hyopsida 



11.-. 



pharyngeal endostyle like that of the tunicates and Amphioxus. During 

 metamorphosis, the endostyle becomes transformed into the thyroid 

 gland of the adult. 



The cyclostomes range in length from a meter to about a third of 

 that. They are geographically widely distributed. Lampreys occur both 

 in fresh and in salt water. The hags are marine. 



Of special interest in connection with the Cyclostomata is the 

 extinct Paleozoic group, Ostracodermi. These fossil and more or less 

 lishlike animals (Fig. 320A) resemble cyclostomes in some striking 

 respects. They had no jaws and probably no paired fins, although in 

 some cases there is doubtful evidence of them. The nasal aperture was 

 median and dorsal. Unlike the cyclostomes, however, they were covered 



CEPHALASPIS- 

 AN OSTRACODERM 



Fig. 320. (A) Cephalaspis, an ostracoderm ; (above) approximately dorsal, 

 (below) lateral aspect. It may have atlinities with cyclostomes. According to 

 W. Patten, it relates vertebrates to arachnids. (B) Climatius, a lower Devonian 

 acanthodian with accessory paired fins: about 3 inches in length. (A, Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company. B, 

 After Traquair and Watson. Courtesy, Romer: "Vertebrate Paleontology," Chi- 

 cago, University of Chicago Press.) 



