310 



COMPARATIVE ANATOMY 



The cavity of the ventricle is unpaired, and neither in Urodela 

 nor Anura shows any trace of a septum, so that the blood passing 

 out from it must have a mixed character (Fig. 249). The ven- 

 tricle is usually short and compressed, but is more elongated in 

 Amphiuma, Proteus, and the Gymnophiona. It is continued an- 

 teriorly into a conus arteriosus, as in Elasmobranchs, Ganoids, and 

 Dipnoans ; this has usually a slight spiral twist, and possesses a 

 transverse row of valves at either end, as well as a spiral fold ex- 

 tending into its lumen.i This holds good for the Axolotl, Amblystoma, 



Fig. 250. 



-He.vkt of Cr;/ptohra iK-lnis japonictis. From the ventral side. 

 Rose.) Ta3 left atrium is out open. 



(After 



S.a, septum atriorum, perforated by numerous small apertures ; L.i', L.i:^, the 

 two pulmonary veins, opening by a single aperture into the left atrium ; O.ai; 

 atrio-ventricular aperture ; 1", 4", the four arterial arches : P.d. andP.s, left 

 and right pulmonary arteries ; />■, truncus arteriosus ; L. Vh, 2^. Vh, left and 

 right atria ; V.s.d and V.x.x, subclavian veins ; V.j.d Sund V.j.s, jugular veins ; 

 r.c.rf, V.c-^, posterior cardinal veins ; V.c.i, postcaval vein. 



Salamandra, Amphiuma, and Siren. In others (e.g., Necturus, 

 Proteus, Gymnophiona), retrogression is seen in a lengthening of 

 the conus, the disappearance of the spiral fold, and the presence of 

 only a single row of valves. 



In Anura, the fold lyiug within the conus extends so far back 

 that no undivided portion of the cavity is left. The consequence 

 of this is that the blood passing into the hindermost pair of the 

 arterial arches — that from which the pulmonary arteries arise — is 

 mainly venous, while the others contain more or less mixed blood 

 (Fig. 249, b) ; for, owing to the spongy nature of the ventricle, there 

 1 This spiral fold corresponds to a series of fused valves. 



