THE HEART AND BLOOD-VESSELS. 167 



The four efferent branchial vessels of each side unite in the 

 dorsal wall of the pharynx to form the aorta : the two aorta3 

 are continued forwards to the head as the carotid arteries, while 

 posteriorly they unite to form the single dorsal aorta, from 

 which branches arise supplying all parts of the body. 



The lungs arise at a very early stage, but are for a long time 

 extremely small and of little functional importance. Each lung 

 receives blood from a branch of the fourth efferent branchial 

 vessel (Fig. 76, ap), and returns it directly to the auricle by a 

 pulmonary vein, VP. As the tadpole increases in size, and the 

 lungs become of greater importance, a septum appears, dividing 

 the auricle into systemic or venous, and pulmonary or arterial 

 cavities. Simultaneously with this, valves are formed in the 

 truncus arteriosus, by which the venous and arterial streams of 

 blood are kept apart to a certain extent. 



At the time of the metamorphosis the gill circulation is cut 

 off, by the establishment of direct communications between the 

 afferent and efferent branchial vessels, and the pulmonary cir- 

 culation becomes of much greater importance than before. 



2. The Heart. 



The heart lies at first (Fig. 69, p. 146) on the under surface 

 of the head, below the floor of the pharynx, above and slightly 

 behind the sucker, and immediately in front of the commencing 

 liver. 



In this region the mesoblast, as in the body generally 

 (Fig. 70, so, Sp), is split into somatic and splanchnic layers, 

 separated by a distinct space. This space becomes the peri- 

 cardial cavity ; the outer or somatic layer of mesoblast forming 

 the wall of the pericardial cavity ; and the inner or splanchnic 

 layer giving rise to the muscular wall of the heart. 



The endothelial lining of the heart is derived from a number 

 of scattered cells, which appear below the floor of the pharynx, 

 and which are formed partly, if not entirely, by direct pro- 

 liferation of the hypoblast cells of the pharyngeal wall and of 

 the liver (cf. Fig. 69). These cells are at first irregularly ar- 

 ranged, but soon become disposed so as to form a tubular lining 

 to the heart, which is for a time closed in front, while its 

 posterior wall is formed by the anterior surface of the liver 

 diverticulum (Figs. 61 and 69). 



