THE ANIMAL KINGDOM 



13 



of vertebrae appear in the form of cartilaginous neural arches. In the 

 genus Bdellostoma there are often as many as fifteen pairs of gill-sHts. 

 There are no scales in the skin, and the teeth are horny. Some species are 

 hermaphroditic. Paired appendages are absent. 



BDELLOSTOMA 



C PETROMYZON 



Fig. 14. — Three characteristic genera of cyclostomes — Bdellostoma, Myxine, and 

 Petromyzon. That they are the most primitive vertebrates is shown in many traits, 

 such as a permanent notochord, absence of paired appendages and jaws, etc. (Redrawn 

 after Dean.) 



The lamprey, Petromyzon, is a familiar genus which undergoes meta- 

 morphosis during its development. Its larval stage is known as Ammo- 

 coetes. Other genera are Myxine and Bdellostoma. 



CEPHALASPIS- 

 AN OSTRACODERM 



Fig. 15. — Cephalaspis, an ostracoderni, appears to have affinities with cyclostomes and 

 has been thought by W. Patten to connect vertebrates with arachnids. 



Class Ostracodermi 



The ostracoderms are fossil forms which, as Stensio and others have 

 shown, resemble cyclostomes in some striking respects. Unlike the latter. 



