747 



Pijia nor Dacff/lefJ/rrf, both of wliicli, though "entirely aquatic", yet 

 "swim actively" (Gadowj, nor in Pclo(l//tes, Avhich is a "good climber" 

 and "jumiis in the manner of the true frogs" (B oui enger), nor in Pe- 

 lobates which "hops along, frog-like", nor finally in the exceedingly 

 active terrestrial frogs and toads. 



Despite the scantiness of the evidence to hand, I feel certain that 

 an extended investigation into the facts would conclusively prove a 

 correlation to exist between activity of the animal and the disappearance 

 of those azygos veins which form indirect connnunications l)etween the 

 post-caval vein and the heart. 



AVitli regard to the posterior sections of the posterior cardinals, 

 which, as we have seen, persist in all Chiropterygia save Mammalia and 

 to wliich persistance indeed, as hope to show in a subsequent jjaper, is 

 chiefly attributable the existence of the renal cardinal meshwork, their 

 replacement by the single median posterior section of the post-caval in 

 Mammalia seems to be due in some manner to the growing importance 

 of the hind limbs. For in A|)teryx, a purely terrestrial bird which, like 

 Mammalia, possesses large perpendicular hind-limbs and at the same 

 time has no renal cardinal meshwork, the two post-posterior cardinals 

 are, as before described, closely apposed in the median line and in fact 

 resemble a single vessel longitudinally slit. 



As I have before said, so far as I am aware from ordinary reading, 

 the above is the first attempt to rationally account for the replacement 

 of the posterior cardinals by the post-caval; it is true that a "rule that 

 all air-breathing animals (Amphibia and Amniota) possess a post-caval" 

 has been vaguely laid down, but no definite reason has yet been assigned 

 to show why the possession of pulmonary organs necessarily implies the 

 presence of a post-caval. Owen, in my opinion, approaches nearer 

 to the truth when he says that "the cardinal veins, essentially those 

 which return the blood from the osseous and muscular segments of the 

 trunk, are largest in the Perennibranchs, and decrease, as the hind 

 limbs acquire more size and power, in the Newts and Land-Salamanders, 

 until, in the tail-less and long-legged Frogs and Toads, the primitive 

 venous trunk of the body is reduced to the condition of the 'azygos' 

 vein in Mammals", implying that increase of limb-power is one cause of 

 the disappearence of the posterior cardinal veins. 



Literature. 



1) Gr. B. Howes, Note on the Azygos Veins in the Anurous Amphibia. Proc. Zool. 



Soc. Part 56. 1888. 



2) W. N. Parker, On the Occasional Persistence of the Left Posterior Cardinal 



Vein in the Frog. Proc. Zool. Soc. Part 57. 1889. 



3) G. A. Boulenger, Tail-less Batrachians of Europe. 



4) Hans G-adow, Amphibia. Cambridge Natural History. 1901. 



