252 DIPNOI. 



the 3rd and 4th have a double series ; while in Protopterus the 5th 

 arch bears one row of gills which are probably an extension of 

 the hindermost row of the 4th branchial. 



In Protopterus there may be three external gill-filaments just 

 above the opsrculum. They are not present in all specimens. 

 They are not found in the adults of the other two genera, though 

 external gills are found in the larva of Lepidosiren (Fig. 134). 



The stomach is a dilatation of the hind end of the oesophagus 

 and the pylorus is at its hind end. The intestine is straight 

 with a spiral valve, and is supported by a ventral as well as by 

 a dorsal mesentery. The hind end of it is a cloaca and receives 

 the urinary and generative ducts. 



The spleen appears to be embedded in the wall of the stomach 

 and there is a pancreas concealed round the bile duct and in 

 the wall of the intestine. There are no pyloric caeca. 



The lung of Ceratodus is single ; it lies on the dorsal side of 

 the alimentary canal, but opens into the ventral wall of the 

 oesophagus, round the right side of which it turns. It contains 

 a central cavity communicating with air-cells in the thickness 

 of its walls. The lungs of Protopterus and of Lepidosiren are 

 very similar, but they are double except in front where they 

 open into the oesophagus. 



The heart shows a tendency to be divided into two lateral 

 halves. This is most completely carried out in Lepidosiren, 

 least so in Ceratodus. The conus arteriosus is well developed 

 and twisted into a spiral (Fig. 131). It contains several longi- 

 tudinal rows of valves ; most of these valves are small, but 

 those of one row are enlarged (Fig. 131, B) and to a certain 

 extent united so as to form a longitudinal valve which in the 

 contraction of the heart divides the conus into two halves, in 

 such a way that the venous blood is directed into the two 

 posterior afferent branchial arteries of each side, while the 

 arterial and the mixed blood is sent into the branchial arteries 

 to the 1st and 2nd branchial arches. The ventral aorta is 

 extremely short as in Amphibia, the branchial arteries arising 

 close together immediately in front of the conus (Fig. 131 A). 



In Ceratodus the sinus venosus is divided into a narrow left portion into 

 which the pulmonary veins open and a larger right portion receiving the 

 systemic veins (Fig. 131, svp, sv). Both of these open into the single auricle, 

 which is partially divided by a septum arising from its dorsal wall (w). 



