71 



of cells, the bodies of which stain with haematoxylin, 

 so that nuclei and cell bodies are not clearly distinguish- 

 able. 



The Vascular System. 



The pericardium {Per., figs. 3 and 4) is a spacious sac 

 which occupies the whole dorsal surface of the body 

 between the posterior wall of the viscero-pedal mass and 

 the posterior adductor. Its anterior wall is closely apphed 

 to the former. Its ventral wall rests on the upper surface 

 of the renal organ. Dorsally and laterally the pericardial 

 wall is also the body wall, and is thin and delicate, except 

 in the median dorsal line where it is produced upwards 

 into a strong ridge, and also on the lateral dorsal borders 

 where the mantle lobes take origin. It is widest in front 

 and contracts as it approaches the posterior adductor. It 

 is nearly filled by the heart consisting of the median 

 ventricle and the two auricles. 



In the animal which has been killed after treatment 

 with cocaine, the ventricle is usually fixed in the condition 

 of diastole and then, together with the two auricles, fills 

 almost the whole pericardial cavity. The ventricle {Ven., 

 figs. 3 and 4) is constricted medially, forming two rather 

 well-marked lateral lobes. The walls are muscular but 

 rather thin. Abundant muscular trabeculsB run across the 

 cavity in all directions. The rectum passes through its 

 cavity, suspending the ventricle in the centre of the peri- 

 cardium. The auricles {Aur., figs. 3 and 4) are triangular 

 in shape, the apices being attached to the ventricle, the 

 bases to the bases of the ctenidia. Their walls are very thin, 

 except at the apical portions, where they are thickened 

 and composed of dense fibrous tissue ; these portions pro- 

 ject into the ventricle ; the openings are horizontal slits 

 bounded by the thickened tissue which forms a pair q|f 



