63 



less spherical ventricle (ve.) Either ventricle sends a 

 very narrow, vessel-like portion towards the mid-line 

 where the two sides unite. In this middle portion there 

 are two semilunar valves (fig. 55) on the dorsal and ven- 

 tral sides, and from this point two vessels emerge. One 

 runs anteriorly, and bending around the posterior adductor 

 muscle, r\ins posteriorly in the mantle. At this star;e, 

 the visceral nmss lias projected but little posteriorly 

 (fig«6), and the second vessel from the heart, somewhat 

 smaller than the other, runs ventrally into the visceral 

 mass. These structures are shown in section in fig. 54, 

 which is a longitudinal section through the median part of 

 the ventricles and aortae in a specimen 4 mm. long. 



In the stage in v;hich the heai-t is developing, the 

 stomach and caecum already occupy most of the visceral 

 mass, and the gills are wide apart. This may account for 

 the wide separation of the tvra halves of the heart. In 

 development posteriorly, the gills advance ahead of the 

 other structures and, accompanying them the two sides of 

 the heart are drawn backwards so as to lie side by side. 

 In the adult (fig. 54) the ventricles (ve. ) have fussd on 

 the midline, except at the posterior end, where the two 



