SEA-FISHERIES LABORATORY. 215 
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 applied 
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 1t 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. 8 and 4) is constricted medially, forming two rather 
well-marked lateral lobes. The walls are muscular but 
rather thin. Abundant muscular trabecule 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 (Awr., figs. 3 and 4) are triangular 
in shape, the apices being attached to the veutricle, 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 of 
