234 



ZOOLOGY 



SECT. 



pharynx by a slit-like aperture, the glottis. Its internal surface is 

 sacculated, and a regularly-arranged series of blind pouches open 

 out of the main central cavity. This lung of Ceratodus corresponds 



morphologically to the air-bladder 

 of Ganoids and Teleosts, but differs 

 from it in its blood-supply, and con- 

 sequently in its function, being sup- 

 plied with venous blood by a special 

 pulmonary artery and acting as an 

 important organ of respiration. 



Blood Vascular System. Co- 

 ordinated with the existence of a lung 

 and distinct pulmonary circulation is 

 a complication in the structure of 

 the heart. The sinus venosus is im- 

 perfectly 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-hand divi- 

 sion of the auricle to the ventricle ; 

 the pulmonary vein, by which the 

 blood is returned from the lung, 

 opens into the left-hand division of 

 the sinus, and its blood reaches the 

 ventricle through the left-hand divi- 

 sion 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 in- 

 terior are eight transverse rows of valves, and its cavity is divided 

 imperfectly by means of an incomplete longitudinal septum. 



The blood-vessels (Fig. 859) 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 (off.) take their origin close together, immediately 

 in front of the conus, so that a ventral aorta can hardly be 

 said to exist. Each branchial arch has two efferent branchial 

 arteries (epi.). A hyoid artery Qiy. art?) is connected dorsally and 

 ventrally with the most anterior of these. The eight efferent 

 vessels unite in pairs to form four epibranchial arteries. The 

 latter unite dorsally to form a main trunk, which combines with 

 the corresponding trunk of the opposite side to form the median 

 dorsal aorta (d, a.). The head is supplied by carotid branches 





FIG. 858. Ceratodus Forsteri. 



Posterior half of the lung with 

 the ventral wall slit up so as to 

 show the interior. (After Giinther.) 



