(54 OEGANIZATION AND DEVELOPMENT OF AXIMALS IN GEXEIIAL. 



from the ventral vessel. The anterior pair, placed behind the mouth, 

 unite beneath the notochord to form the root of the median body 

 artery (descending or dorsal aorta) which receives the hinder succes- 

 sive pairs of lateral vessels. This dorsal artery gives off branches to 

 the muscles of the body wall and the viscera, from which the venous 

 blood in part is returned to the ventral pharyn- 

 geal vessel; part of it, however, before reaching 

 the latter, traverses a capillary network in the 

 liver. 



t \l\J^ From the- hinder part of the ventral pha- 



ryngeal vessel there is developed, in the higher 

 Yertebrata, the heart, which at first has the 

 shape of an S-shaped tube, but later acquires 

 a conical form and becomes divided into auricle 

 and ventricle. The former receives the blood 

 returning from the body and passes it on into 

 the more powerful ventricle, from which arises 

 an anterior vessel, the ascending or cardiac 

 aorta, presenting a swelling at its root, known 

 as the aortic bulb. This vessel leads, by means 

 of lateral vascular arches, the arterial arches, 

 into the dorsal aorta, which passes backwards 

 beneath the vertebral column, and supplies the 

 body. Yalves placed at the two ostia of the 

 | ventricles regulate the direction of the blood 



stream ; and they are so arranged as to prevent 

 ^onhe^atc^rTysl'm an y Backward flow of blood from the cardiac 

 ofanOiigochseteworm aorta into the ventricle in diastole, and from 



(Ssenuris) (after Ge- . . .. . . . . 



genbaur). in the dor- the ventricle into the auricle in systole,. 



sal vessel the blood j n consequence of the insertion of the respi- 



moves from behind 



forward ; in the ven- ratory organs on to the system oi the arterial 

 f re arches, the latter, and at the same time the 

 rows). H, heart-like structure of the heart, assumes various degrees 

 MertLsse^ nSVerSe of complication. In fishes (fig. 57), four or five 

 pairs of gills are inserted in the course of the 

 arterial arches, which break up into a respiratory capillary net- 

 work in the branchial leaflets. From this network the arterialised 

 blood is collected into efferent branchial arches, the branchial veins, 

 corresponding each to a branchial artery ; and these unite to form 

 the dorsal aorta. In such cases the heart remains simple, and 

 receives venous blood. 



