ANATOMY OF VERTEBRATES. 9 



Subclass V. Monopnoa. — Endoskeleton ossified ; exoskeleton 

 in most as horny scales, in some as bony scutes; one occipital 

 condyle ; vomer usually single ; trunk-ribs long and curved. Brain 

 with predominant prosencephalon. Labyrinth with both fenestra 

 vestibuli and fenestra rotunda ; a tympanum in most ; lungs ; 

 heart with two auricles, and with the ventricle more or less 

 completely divided. Testes with ducts and intromittent organ. 

 Ovaria with detached oviducts. Ova successively developed, 

 impregnated with copulation. An amnios and allantois. No 

 metamorphosis. 



§ 9. Orders of HJEMATOCRYA. 



Subclass I. Order I. CIRROSTOMI. 



Body compressed ; mouth a longitudinal fissure with sub-rigid 

 cirri on each side. Pulsating vessels or sinuses in place of heart. 

 Blood pale ; free pharyngeal branchial filaments, and a branchial 

 dilatation of the oesophagus. 



Gen. Br anchio stoma. Ex. Lancelet. 



Order II. CYCLOSTOMI. 



Body cylindrical ; heart distinct ; branchial artery without 

 bulb ; branchia? sacciform, with external spiracles, six or seven on 

 each side, blood red. Mouth subcircular, suctorial, but longitu- 

 dinal when closed. Olfactory sac communicating with, or produced 

 into, a canal. 



Gen. Myxine. Ex. Hag-fish. 



Petromyzon. Ex. Lamprey. 



Subclass II. A. Arterial bulb with one pair of valves; optic 

 nerves decussating ; vertebras biconcave. 



Order III. MALACOPTERI. 



Skin, in most with cycloid scales, in a few with ganoid plates ; 

 rarely naked. Fins supported by rays, all of which (save the first 

 in the dorsal and pectoral, in some) are ' soft,' or many-jointed ; 

 a swim-bladder and air-duct ; peritoneal outlets in many. 



