VERTEBRATA. 351 



There is a closed circulatory system formed in the mesoblast consisting 

 of a central rhythmically contractile heart, a set of efferent vessels or arteries, 

 and of afferent vessels or veins the two sets connected by the smallest 

 vessels or capillaries. The heart is primitively a specially developed section 

 of the subintestinal vein in the region of the throat, but in Amniote 

 Vertebrata (? Rcptilid) it is formed by the coalescence of two vessels directly 

 continuous with the vitelline veins. It consists primitively of a sinus 

 venosus, lost in adult Aves and Mammalia, an auricle and a ventricle sepa- 

 rated by valves ; a truncus arteriosus divisible into a conus arteriosus, con- 

 taining many valves and a bulbus arteriosus (or aortae), which is valveless. 

 The auricle lies dorsally to the ventricle and truncus, and anteriorly to the 

 former. It is divided by a septum into a right and left auricle from 

 Amphibia upwards. The ventricle is similarly divided in Aves and Mam- 

 malia. The complete division of the ventricle which exists in Crocodilia is 

 perhaps not homologous with that of higher Vertebrata. The conus is 

 found in all Ichthyopsida, except Teleostei, in which it aborts ; the bulbus 

 only in Pisces 1 . The embryonic truncus (i.e. conus + bulbus) is broken up 

 by an internal septum in all Sauropsida and Mammalia into the roots of 

 the great vessels, i. e. the aorta and pulmonary artery. The roots of these 

 vessels are guarded in Sauropsida by two, in Mammalia by three, semilunar 

 valves. The heart lies in a special section of the coelome the pericardium. 

 The part of the coelome dorsal to the pericardium is aborted in Pisces. 

 In other Vertebrata it enlarges, and the lungs grow out into it. The bulbus 

 is primitively continued forwards as the ventral aorta as far as Meckel's 

 arch, and gives off to the right and left paired aortic arches, which correspond 

 to Meckel's arch, the hyoid and following branchial arches. The mandi- 

 bular aortic arch is aborted in growth, and is sometimes not developed 

 (Amphibia) ; the hyoidean persists in some Pisces. In Sattropsida and 

 Mammalia only three branchial aortic arches are ever developed, but four 

 at least in most Ichthyopsida. Some or all of these aortic branchial arches, 

 and in some instances the hyoidean also, supply the internal or external 

 gills of Ichthyopsida, in the former case being resolved into a branchial 

 artery and vein. The dorsal ends of the primitive aortic arches unite on 

 each side into a vessel which gives off forwards to the head one or two 

 branches, the carotid arteries, external as well as internal, and then runs 

 backwards beneath the notochord parallel to its fellow with which however 

 it fuses, sooner or later, to form the subvertebral aorta of the adult. This 

 aorta gives off vessels to the fore-limbs, or the subclavian arteries ; a vessel 

 to the yolk sac, the vitelline artery (the persistent mesenteric artery) ; one or 

 two iliac arteries to the hind-limbs which are connected with the allantois 

 in foetal Sauropsida and Mammalia, by the umbilical arteries, the remnants 



1 The distal rows of valves of the conus persist in Teleostei, the proximal apparently, in 

 Sauropsida and Mammalia. The Amphibia have both a distal and proximal row in their conus. 



