tricle, but no spiral valve. There is a distal rins: of valves 

 in Cryplohranchus. The distal end of the truncus is much 

 expanded in some salamanders, as a bulbus arteriosus; this 

 term is also used sometimes for the conus area of the trun- 

 cus. In Salamandra there are proximal and distal series of 

 valves and a very small spiral valve. 



The amphibian heart differs from the reptilian or mam- 

 mal heart in that the sinus venosus is retained. In the previ- 

 ous groups it has been reduced or lost. In the amphibian 

 heart one can observe the trabeculae carnae, the strands of 

 muscle tissue making up the muscular walls. These trabecu- 

 lae are best observed in the atrial walls where they are 

 separated by membranous areas. 



Embryo/og/ca/ development The amphibian heart devel- 

 ops much like that of the preceding groups. There is a sig- 

 moid flexure in the vertical (sagittal) plane. The sinus 

 venosus and atrium are dorsal, and the ventricle postero- 

 ventral with the truncus leads forward from it. A fenes- 

 trated interatrial septum appears which extends toward the 

 atrioventricular opening. The atrioventricular cushions 

 margin this opening above and below and give rise to the 

 flap valves extending into the ventricle. The pulmonary 

 veins develop along with the interatrial septum. At first they 

 open into the sinus venosus, but this opening moves to the 



left so that an entrance into the left auricle is effected. In 

 the plethodontids (lungless salamanders), neither interatrial 

 septum nor pulmonary veins develop. In the aquatic types, 

 the septum remains incomplete and fenestrated or degene- 

 rates in the adult. 



Choanate fishes 



The heart of Prolopterus or Leptdosiren is four-chambered 

 (Figure 1 1-5). The sinus venosus leads by way of the broad 

 opening into the auricular chamber which is expanded on 

 either side of a thin, perforate septum. The blood from the 

 sinus enters to the right of the septum, the blood from the 

 pulmonary vein to the left. There is a large opening below 

 the auricles leading to the ventricle; in this opening is a large 

 cushion mass, the ventral atrioventricular cushion. This 

 cushion continues into the ventricle as a partial interven- 

 tricular septum. Anterior to the interventricular septum is a 

 single ventricular cavity. 



The truncus is large with a sharp bend in it. There are 

 three rows of proximal valves. The spiral valve begins 

 ventrally, curves around the right side to the dorsal wall, 

 and then extends forward along the dorsolateral wall to 

 the anterior end of the truncus. The spiral valve is the 

 equivalent of the left dorsolateral fold observed in the reptile 



V^- 



, 4th aortic arches 



5th and 6th arches 

 dium 5th and 6th arches 



entrolcteroi fold 



proximal valves and 



nterventricular septum 

 and cushion 



.ventricle 



PROTOPTERUS 



NEOCERATODUS 



Figure 1 1 -5. Hearts of Propferus and Neocerofodus as seen from below and with the truncus 

 opened to show the valves. (In part, after Goodrich) 



344 • THE CIRCULATORY SYSTEM 



