ORGANS OF CIRCULATION. 



181 



length to the pericardium ; but it rapidly increases in length, 

 and as a result becomes twisted into an S-shaped tube, and with 

 this twisting the heart becomes differentiated. In the S are 

 developed two chambers, an anterior ventricle and a posterior 

 atrium or auricle, the tube between these remaining smaller, 

 the atrio- or auriculo-ventricular canal. In this twisting only 

 the posterior portion of the tube takes part, and the atrium comes 

 to have the more dorsal position (Fig. 194). 



The anterior straight portion of the tube gives rise to the 

 truncus arteriosus and the ventral aorta. In the truncus region, 

 which immediately adjoins the heart, two parts may be differen- 

 tiated, a posterior conus arteriosus, containing on its inside 

 membranous valves preventing any backward flow of the blood, 

 and an anterior and muscular bulbus arteriosus. Behind the heart 

 the two omphalomesaraic veins unite to form a cavity, the sinus 

 venosus, into which, later, other veins entering the heart come 

 to empty. Valves soon arise in the auriculo-ventricular canal, 

 and a little later other valves are formed at the opening of 

 the sinus into the 

 atrium. These 

 valves are fleshy 

 folds which prevent 

 any backward flow 

 of the blood. 



In its earlier 

 stages the heart lies 

 in the region of the 

 gill slits (Fig. 122); 

 but as the animal 

 increases in age 

 there is a relative 

 shifting of parts, so 

 that the heart 

 comes to lie behind 

 the gills, and in 

 many forms is removed some distance from them. 



The conditions so far described are permanent in fishes, and 

 also occur in the younger stages of all higher forms, with the 



FIG. 195. Early stage in the development of the 

 heart in the tern (^Sterna'), a, anterior end of the ali- 

 mentary canal ; c , axiom, later cut off as pericardium ; 

 e, epidermis; le, left endothelial cavity; , notochord; 

 /, wall of somatoplure, which later gives rise to muscles 

 of the heart; re, right half of heart; so, somatoplure; 

 sp, splanchnoplure; vm, ventral mesocardium. 



