THE ANIMAL KINGDOM 



23 



obscured in the higher. The heart is ventral and may be either two, 

 , three, or four-chambered. 



Some :>^,ooo species are known. 



Vertebrates are divided into seven classes. 



Class Cyclostomata 



Cyclostomes are the round-mouthed lamprey eels and hag-fishes. 

 They have a persistent notochord, lack a biting jaw, and the beginnings 

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

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



BDELLOSTOMA 



VGILL APERTURES (?) 

 ■""''' " ■ C. PETR0MY2DN 



Fig. 16. — 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.) 



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

 hermaphroditic. Paired appendages are absent. 



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. 



Class Ostracodermi 



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

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

 however, their heads were covered by heavy bony armor. Like the lam- 

 preys they lacked jaws and paired appendages. As in cyclostomes the 

 nasal aperture was median and dorsal in position. It has been asserted 

 but not demonstrated that the ostracoderms are the ancestors of carti- 

 laginous fishes, which are consequently assumed to have lost their heavy 

 body exoskeletons. Most morphologists, however, consider ostracoderms 



