244 



ZOOLOGY 



. SECT. 



hyoidean gill or pseudobranch is present as well. The lung (Fig. 

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

 with a muscular chamber or vestibule opening into the oesophagus 



on its ventral side by a slit-like aper- 

 ture 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 Ceratodus corre- 

 sponds 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 Polypterus) and acting as 

 an important organ of respiration. 



Blood- Vascular System. Co- 

 ordinated with the existence of a 

 lung and distinct pulmonary circula- 

 tion, is a complication in the struc- 

 ture of the heart. The sinus venosus 

 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-hand division of the auricle to the ventricle ; 

 the pulmonary 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 longi- 

 tudinal rows of valves, one of which is modified to form an incom- 

 plete 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 with the last two. 



The blood-vessels (Fig. 920) 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.) 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. A hyoid 



FIG. 919. Ceratodus forsteri. 



Posterior half of the lung with the 

 ventral wall slit up so as to show 

 the interior. (After Giinther.) 



