ix INTRODUCTION TO THE VERTEBRATA 293 



A well-developed body-cavity coelomic in its nature is present but 

 it has lost the segmentation which is so pronounced a feature in the 

 annelids. 



The renal organs are nephridial tubes which, however, open not directly 

 to the exterior but into a longitudinal duct on each side which in turn 

 opens at its hinder end into the terminal part of the alimentary 

 canal. 



The wall of the pharyngeal region of the alimentary canal is per- 

 forated on each side during at least the young state if not throughout 

 life by a series of gill-clefts, the lining of which forms in the lower 

 aquatic vertebrates the main organ for respiratory exchange with the 

 surrounding medium. 



The fundamental plan of the blood-system of vertebrates is similar 

 to that of annelid worms a dorsal and a ventral longitudinal vessel 



ucl 



ua 



H h.l 



FIG. 120. 



Diagram illustrating the arrangement of the main blood-vessels in the vertebrata. a, Anus ; 

 a.a.l, first aortic arch ; a.r, aortic roots ; d.a, dorsal aorta ; H, heart ; ent, enteron ; h, hepatic 

 vein; h.p, subintestinal vein ; I, liver; m, mouth; v.a, ventral aorta; v.c.l and VI, visceral clefts. 



being connected by hoop-like vessels round the sides of the alimentary 

 canal. The main longitudinal vessel on the neural side (i.e. in this case 

 the dorsal side) is the, dorsal aorta (Fig. 120, d.a)} the ventral vessel is 

 known as the subintestinal vein in its hinder portion (h.p.), while its front 

 part lying beneath the pharynx is the ventral aorta (v.a). The portion of 

 ventral vessel immediately behind this is enlarged and forms the heart (H). 

 The hoop-like vessels are distinct only anteriorly where they form the 

 aortic arches (a.a) passing from ventral aorta to dorsal aorta between 

 consecutive gill-clefts. 



STRUCTURE OF THE VERTEBRATA AS ILLUSTRATED BY THAT OF 

 AN ELASMOBRANCH FISH 



A general idea of the organization of a vertebrate is best got by the 

 study of one of the comparatively archaic shark-like fishes such as the 



