746 



In animals which are not very active and in which in consequence 

 there is not a very great flow of blood to render a median channel of 

 much greater advantage than lateral channels, nor flexion of the trunk 

 to cause tensions in these laterally situated veins ^ and thereby to render 

 still more advantageous the adoption of a median channel, these lateral 

 channels persist, but in all active animals they disappear, at any rate 

 in the capacity of affording alternative courses for the return 

 to the heart of the blood contained in the post-caval. Thus, 

 to give the few examples which are obtainable, thougli the Salamanders 

 are thoroughly terrestrial yet they are extremely inactive in their move- 

 ments and correlated with this inactivity. "Hoch s tetter states that 

 the hepatic portion of the post-caval remains undeveloped exceptionally 

 in the Salamander, in which case either one or the other cardinal be- 

 comes correspondingly enlarged" (Howes). Speaking more generally, 

 "'the anterior part of both posterior cardinals persists in Urodeles [not 

 in some of the more active species] and in Bomhinator^ as the paired 

 azygos vein, and this may exceptionally be present on one or both sides 

 in other Anurans" (Wiedersheim). In the two sjDCcimens of Bombi- 

 iuitor (described by B oui enger [3] as being "thoroughly aquatic during 

 its periods of activity" and as only "proceeding by small leaps on land") 

 examined by Howes, the azygos veins as just stated, joined the poste- 

 rior cardinals (post-caval); Howes also found this fairly frequently to 

 be the case in Alytes obstetricans (in one specimen out of five), which 

 "is nocturnal and slow in its movements . . . and progresses mostly 

 crawling, but sometimes by short leaps" (B oui enger), and once in 

 Biscoglossus, which however "resembles the true frogs in the quickness 

 of its movements . . . and is found in or about water" (B oui enger). 

 Howes concludes that "the character is fairly distinctive of the Disco- 

 glossidae", which, as a group, contains the most primitive and inactive 

 of Anura. On the other hand, no azygos veins are to be found in Hyla, 

 which is a "poAverful jumper and expert acrobat" (Gadow [4]), nor in 



5 These flexions of the trunk which are involved, however slightly, in the or- 

 dinary activity of the animal inevitably cause any line situated at right angles to the 

 median straight longitudinal axis of the animal to become inclined to that axis. 

 How taking the junctions of the posterior cardinals to the Cuvierian ducts as two 

 ])oints, situated one on either side of the median line, it is evident that the line join- 

 ing these constitutes such a transverse line as that just mentioned. Hence it follows 

 that on the occurrence of flexion of the trunk to say the left, a vein attached ante- 

 riorly to the base of the right subclavian and posterioi'ly at the level of the kidneys 

 to the median post-caval must undergo tension and therefore offer obstruction to the 

 passage of blood; i.e. lead to the enlargement of the median continuation of the post- 

 caval. It is pretty evident I think that in the course of many generations activity 

 of the animal would, in the manner just indicated, lead to the sole persistance of this 

 anterior median channel. 



