742 



persist, and not only persist but become Avell-developed, since the mass 

 of the venous blood, prohibited from returning to the heart by way of 

 the constricted lateral channels situated between the coelomic epithe- 

 lium and the external muscles , will be diverted into channels that are 

 permanently open (Fig. 4 B and C). Confirmatory of this explanation of 

 the median situation of the vascular channels (both dorsal and ventral) 

 in the vertebrata is the fact that the tendency of these vascular channels 

 to assume a median position is strictly correlated with the degree of 

 flexion experienced by the portion of the body containing them. For 

 example, in fishes the dorsal posterior cardinals diverge anteriorly where 

 fiexion is smallest and converge posteriorly as flexion increases, finally 

 fusing to form a single vessel in the tail where flexion is at a maximum. 



Trunk 



Trunk 



Pelvis 



Tail 



Tail 



ICHTHYOPTERYGIA 



CHEIROPTERYGIA 



Fig. 5. The above diagrams roughly illustrate the direct relation existing between 

 appi'oximation of the veins to the median line and intensity of flexion in dififerent 

 parts of the body in Icìitlìyopterygia and Ci/eiroptrrygia. The veins are represented 

 by the black lines; the intensity of flexion is roughly proportional to the widtli of the 

 shading in any given region. 



I See fig. 5. Icldlnjopterijgia.) Now in vertebrates above fishes, though 

 flexion of the trunk and tail still occurs, yet it is most important to notice 

 that the flexions of these two portions of the body in these higher verte- 

 brata are distinct from each other: the trunk and tail do not bend to and 

 fro as one body, as in fishes, but act independently, with a nodal region, 

 or region of no flexion, separating them — the pelvic region. Flexion of 

 the trunk and tail originates from one centre so long as the limbs are 

 not locomotor in function or but little so, but when^ as in terrestial 

 animals, the limbs exchange their balancing function for a locomotor 



