43 



is asymmetrical in shape. The walls of this chamber are 

 muscular and, unlike the dark coloured branchial hearts, 

 of a whitish colour. The inner surface is produced into 

 numerous branching and interlacing fleshy pillars, which 

 are bathed in arterial blood (PL VI, fig. 45). The cavity 

 of the ventricle is incompletely divided into two chambers 

 by a fleshy partition which probably aids in ensuring the 

 distribution of blood through all three aortae during the 

 period of systole. This is triangular in shape, and as 

 the dorsal and anterior edges are attached to the 

 corresponding walls of the heart, while the basal side 

 hangs free, an incomplete vertical septum is thus formed. 

 (PL VI, tigs. 47 a and b, tr. s., and fig. 45). The right 

 auricle leads into the right chamber, and the left auricle 

 similarly into the left chamber of the ventricle. The 

 anterior aorta is given off from the right anterior dorsal 

 corner of the heart, while the abdominal aorta leads out 

 of the ventral surface of the left chamber and the genital 

 aorta from the posterior dorsal wall of the same (figs. 

 45, 46 and 47). Two semi-lunar valves guard the entrance 

 of each auricle into the ventricle. The free edge of each 

 is directed inwards into the ventricle (fig. 48, an. v. and 

 au x l\). Consequently, at the moment of diastole they 

 open and allow blood to enter the ventricle from the 

 auricles, and at this time the blood in the two chambers 

 of the ventricle can mix freely. However, at the moment 

 of systole the valves close and prevent the reflux of blood 

 into the auricles. The ventricle is now completely 

 divided into two chambers by the partition, and the 

 blood from the right chamber is forced up the anterior 

 aorta, while the blood in the left flows into the abdominal 

 and genital aortae. The anterior aorta also has two 

 smaller semi-lunar valves at its base. These have their 

 free edges turned towards the aorta. They close during 

 diastole, and open during systole. 



