<>4 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



\ 



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 venovis 



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 



} ackward fl f blood f th car( }i ac 



J 



aorta into the ventricle in diastole, and from 

 . , . . , . 



the ventricle into the auricle in systole. 



j n consequence of the insertion of the respi- 

 ratory organs on to the system ot the arterial 

 arcnes the latter, and at the same time the 

 structure of the heart, assumes various degrees 

 f comp }i ca ti O n. 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. 56.-Anterior part 

 of the vascular system 

 ofanOHgochseteworm 

 (Ssenuris) (after Ge- 



genbaur). in the dor- 

 sal vessel the Hood 



moves from behind 



forward ; in the ven- 

 trai vessel from before 



backwards (see ar- 

 rows). //, heart-like 

 dilated transverse 



lateral vessels. 



