AIK-BLADDER. 



149 



dically or in an auxiliary manner. The ductus pneumaticus 

 is a membranous bronchus, entering the ventral side of the 

 cesophagus, and provided at its entrance with a glottis. In 

 Ceratodus (Fig. 65) the lung is still a single cavity, but with 

 a symmetrical arrangement of its internal pouches; it has 

 no pulmonal artery, but receives branches from the arteria 

 cosliaca. Finally, in Lepidosiren and Protopterus the lung is 

 completely divided into lateral halves, and by its cellular 

 structure approaches most nearly that of a reptile; it is 

 supplied with venous blood by a true pulmonary artery. 



J- 



Fig. 66. — Heart of Lepidosteus osseus. 

 I. External aspect. II. Conus arteriosus opened. 

 a, Atrium ; b, Conus arteriosus ; v, Ventricle ; h, Branchial artery for 

 3d and 4th gill ; k, for the second ; I, for the first ; m, branch for 

 the opercular gill ; d, Single valve at the base of the conus ; 

 e-g, Transverse rows of Ganoid valves. 



