272 COMPARATIVE ANATOMY 



we have a ccelomic cavity lying on either side of a median 

 double tube, the heart. The layer of splanchnopleur surround- 

 ing the endothelial tube gives rise to the muscular tissue of the 

 heart and to that part of the endothelial lining of the pericardial 

 cavity which invests the heart. The two ccelomic cavities 

 become the pericardial cavity. They are at first separated 

 from one another by the vertical partitions attached to both 

 the dorsal and the ventral side of the heart, but after a time 

 the ventral partition disappears and the heart is suspended 

 only by the dorsal partition or mesocardium, and the two 

 pericardial cavities coalesce ventrally. The communication 

 between the pericardial cavity and the general coelom behind 

 is obliterated on the formation of the great veins bringing blood 

 to the heart. These are two large vitelline veins running along 

 the sides of the liver and bringing blood from the yolk-mass, 

 and the ductus Cuvieri bringing blood from the dorso-lateral 

 regions of the body. These veins, uniting just before they 

 reach the heart, form the sinus venosus. The ductus Cuvieri 

 of each side receives an anterior and a posterior cardinal vein. 

 The former persist in the adult as the superior venae cavse, 

 but the posterior cardinals undergo great modifications, their 

 anterior ends atrophy on the formation of the inferior vena 

 cava, and their posterior ends share in the formation of the 

 renal portal veins. Thus the tadpole passes through a stage 

 in which its venous system is arranged on the same plan as 

 that of the dogfish. 



The arterial system during the time that the tadpole is 

 breathing by gills, is even more remarkably piscine in its 

 arrangement. The heart is continued forwards beneath the 

 pharynx as the cardiac aorta — the future truncus arteriosus. 

 After a short course this vessel divides into right and left 

 branches, and these immediately subdivide into vessels 

 running up the gill-arches. By the time that the three pairs 

 of external gills are established, we find the arrangement 

 shown in fig. 6S,A. The truncus has divided into three pairs of 

 afferent branchial arteries. One pair passing up the first 

 branchial arch, supplies the first pair of external gills. The 

 second and third pairs similarly pass up the second and third 

 branchial arches, and supply the second and third pairs of 

 external gills. There are no afferent arteries for the mandibu- 

 lar and hyoid arches, and as yet there is no afferent artery 



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