270 



CLASSIFICATION OF VERTEBRATES. 



FIG. 271. Tooth 

 of ceratodan, Sageno- 

 dus, after Woodward. 



smaller pair on the vomerine region, while the lower jaw has 

 a pair on the splenial region. The alimentary canal is nearly 

 straight, and is char- 

 acterized by the pres- 

 ence of a well-devel- 

 oped spiral valve 

 (Fig. 40) in the in- 

 testine. Behind, the 

 intestine empties into 

 a cloaca, which also receives, besides the 

 urogenital ducts, median or paired pori 

 abdominales. There are three (Protop- 

 terus) or four (Ceratodns) 1 pairs of in- 

 ternal gills, and besides, in the 

 former, external gills (Fig. 269). 

 Besides these, there are present in 3 

 each swim-bladders which also have 

 respiratory functions. In Cera- 

 todus this lung is single, in Pro- 

 topterus it is paired ; but in 

 both its duct or ducts arise 

 from the ventral surface of the 

 pharynx. Internally these or- 

 gans are sacculated, while the 

 blood comes to it by true pul- 

 monary arteries, which arise 



either (Ceratodus} from the pOS- FIG. 272. Heart and anterior part 

 terior branchial, or (Protopterus) of the lun e s of Ceratodus, after Rose. 



, . .. a, aortic arches and auricle; f, post- 



from the radices aortae. cardina , vein . ^ conus; ^ hepatic 



The heart has both the sinus veins; /, lung; j, jugular vein; ji, in- 



and the atrium partially divided ferior i u e ular vein '> oe > oesophagus ; /, 



. . . .' . , ,. . , , pulmonary arteries; s, subclavian vein 



into right and left halves by an , . 



and sinus venosus. 



incomplete septum, thus fore- 

 shadowing the conditions found in the amphibia, while a true 

 atrio-ventricular valve is lacking. The conus is spirally twisted, 

 and contains several rows of valves, and in Ceratodus is partially, 

 in Protopterus completely, divided into venous and arterial halves. 



1 Ceratodus also has a hyoid pseudobranch (p. 23). 



