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 (J'rotop- 

 terus) or four (Ceratodiis) '^ 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- 

 todics 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 (^Ceraiodus) from the pOS- Fig. 272. Heart and anterior part 



terior branchial, or (Protoptenis) °f ''^^ '""S^ »* Cemtodus, after Rose. 



^ , ,. ^, aortic arches and auricle: c, post- 



irom the radices aortae. .i- 1 ■ ; i, .- 



cardinal vein ; lo^ conus ; //i', hepatic 

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



and the atrium partially divided f«™"^ jugular vein; oe, oesoph^; /, 



. , . 1 ^ 1 1 f ^ 1 1 ' 1 pulmonary arteries ; 5, subclavian vein 



into right and left halves bv 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 Ceratodiis is partially, 

 in Protoptenis completely, divided into venous and arterial halves. 



I Ceratodiis also has a hyoid pseudobranch (p. 23). 



