THE HEART AND CIRCULATORY SYSTEM 111 



Rana and Bufo it is distributed mainly to the carotid and right 

 systemic and never goes to the pulmo-cutaneous arch. These 

 variations among the Anura are quite probably associated with 

 differences in their respiratory mechanisms. For example, 

 Xenopus is known to depend more on cutaneous respiration 

 than the other two genera and associated with this it has been 

 found that over a half of the total ventricular output passes into 

 the pulmo-cutaneous circulation and especially that part which 

 goes to the skin. In urodeles, where cutaneous respiration is also 

 of great importance, there is far less separation of the blood 

 from the left and right auricles in the conus largely because of 

 the absence of a spiral valve. The probable mechanism of this 

 valve in Rana and Bufo is illustrated diagrammatically in fig. 32, 

 where it is suggested that the blood from the ventricle makes a 

 clockwise spiral even before it enters the base of the conus. This 

 has been observed from cine-injection experiments and it is also 

 present in forms without a spiral valve. It seems to be a property 

 of the hydrodynamics and the contractile mechanism of the 

 ventricle in these forms. Measurements of the oxygen tension 

 or the relative volumes passing into the different arches have 

 not been made in Rana or Bufo. Until these are available it must 

 be realised that the terms oxygenated and de-oxygenated blood, 

 although convenient, may be misleading. In the urodele, 

 Amphiuma, it has recently been shown that the blood in the 

 systemic arch is richer in oxygen than that in the pulmo-cutan- 

 eous arch, but this difference is reduced at low blood pressures. 

 During metamorphosis the tadpole passes from a fish-type 

 circulation to that of the adult. At one stage it has external gills 

 which are replaced by internal gills. Various parts of the 

 ancestral pattern of six aortic arches persist in Amphibia; for 

 example, the ductus caroticus, which connects the third and 

 fourth aortic arches, persists in Apoda (legless Amphibia) and in 

 some adult urodeles such as Triton. The ductus arteriosus (ductus 

 Botalli), which connects the pulmonary and systemic arches, is 

 lost in adult Anura but is found in most Apoda and Urodela. 

 The urodeles also show the presence of a persistent 5th aortic 

 arch which is entirely lost in all other tetrapod adults. 



