2/0 



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 (Ceratodus) 1 pairs of in- 

 ternal gills, and besides,, in the 

 former, external gills (Fig. 269). 

 Besides these, there are present in 

 each swim-bladders which also have 

 respiratory functions. In Cera- 

 todus this lung is single, in Pro- 

 toptcrns 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 (Ceratodlis) from the pOS- FIG. 272. Heart and anterior part 



terior branchial, or (Protopterus) of the lun s of C*r<" odus * after Rose - 



r ,. a, aortic arches and auricle; c, post- 



from the radices aortae. cardinal vein . ^ conus; //7 , hepatic 



The heart has both the sinus veins: /, lung; /, jugular vein; jt, in- 



and the atrium partially divided ferior jugular vein; oe, oesophagus; /, 



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



into right and left halves by an and sinu / 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 Ceratodns is partially, 



in Protoptems completely, divided into venous and arterial halves. 



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



