XIII 



PPIYLUM CHORDATA 



251 



hyoidean gill or pseudobrancli is present as well The lung (Fig 



908) is an elongated median sac connected by a pneumatic duct 



with a muscular chamber or vestibule opening into the oesophao-us 



on its ventral side by a slit-like "^ 



aperture or glottis. The internal 



surface of the lung is sacculated, 



and a regularly-arranged series of 



blind pouches opens out of the main 



central cavity. This lung of Cera- 



todus corresponds morphologically to 



the air-bladder of Ganoids and 



Teleosts, but differs from it in its 



blood-supply and consequently in 



its function, being supplied with 



blood by a special paired pulmonary 



artery (as is also the case in Poly- 



pterus) and acting as an important 



organ of respiration. 



Blood-Vascular System. — Co- 

 ordinated with the existence of a 

 lung and distinct pulmonary circu- 

 lation, is a complication in the struc- 

 ture of the heart. The sinus mnosus 

 is imperfectly divided into two parts, 

 and the cavity of the auricle is divided 

 into two by an incomplete septum 

 in the form of a ridge. The venous 

 blood enters the right-hand division 

 of the sinus venosus and passes 



thence through the right-band division of the auricle to the 

 ventricle ; the piolmonary vein, by which the blood is returned 

 from the lung, passes through the sinus, and its blood reaches 

 the ventricle through the left-hand division of the auricle. 

 There are no auriculo-ventricular valves guarding the opening 

 between the auricle and the ventricle. A contractile conus 

 arteriosus is present, and has a remarkable spirally-twisted 

 form ; in its interior are four longitudinal rows of valves, one of 

 which is modified to form an incomplete longitudinal septum. The 

 channel on the left side of this septum, which receives the blood 

 of the pulmonary vein, is in communication in front with the first 

 two aortic arches (afferent branchials), that on the right witli 

 the last two. 



The blood-vessels (Fig. 909) present an arrangement which is 

 intermediate in some respects between that which has been already 

 described as observable in the Elasmobranchs and that which will 

 be found to characterise the Amphibia. The four afferent 

 branchial arteries (aff.) taketheiroriginclose together, immediately 



Fig, 



CeratoduB forsteri. 



Posterior half of the lung with the 

 ventral wall slit up so as to show 

 the interior. (After Giinther.) 



