(54 OEGjLNIZATION AXD DEVELOPMENT OF AXIMALS IN GENERAL, 



t 



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. 



From the hinder part of the ventral pha- 

 ryngeal vessel there is developed, in the higher 

 Vertebrata, 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. Valves placed at the two ostia of the 

 ventricles regulate the direction of the blood 

 stream ; and they are so arranged as to prevent 

 any backward flow of blood from the cardiac 

 aorta into the ventricle in diastole, and from 

 the ventricle into the auricle in systole. 



In consequence of the insertion of the respi- 

 ratory organs on to the system of the arterial 

 arches, the latter, and at the same time the 

 structure of the heart, assumes various degrees 

 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. 



FIG. 50. Anterior part 

 of the vascular system 

 of an Oligochffite worm 

 (Sanuris) (after Ge- 

 genbaur). In the dor- 

 sal vessel the blood 

 moves from behind 

 forward ; in the ven- 

 tral vessel from before 

 backwards (see ar- 

 rows). H, heart-like 

 dilated transverse 

 lateral vessels. 



