SEA-FISHERIES LABORATORY. 407 
Al. c. 5, and fig. 45), as in the majority of lamellibranchs. 
The wall of the ventricle is composed of a layer of 
epithelial cells on the outer side, resting on a delicate 
basement membrane of connective tissue. Internal to, 
and lying against this latter, are the muscles, which run 
across the ventricle in all directions, imbedded in a 
eranular matrix which sheathes the bundles of fibres. 
There appears to be no striation on these fibres. 
The Auricles are two large chambers, one on each 
side of the ventricle, and having a brown tint. ‘heir 
shape is roughly conical, the base being uppermost and 
communicating on each side with the ventricle, and 
opening by their narrower ends into the vein bringing 
blood from the gills and mantle to the heart. In Anodon 
and many other lamellibranchs the triangular auricles 
communicate with the efferent branchial vessel by the 
whole length of the base, and the narrow end opens into 
the ventricle. The position in Pecten may indicate the 
primitive molluscan arrangement, with posterior ctenidia. 
The walls of the auricles are not smooth like those of 
the ventricle, but are raised all over into papillae, 
representing depressions of the inner surface (fig. 13). 
This peculiar papillated appearance is best seen when the 
auricle is distended by blood or injection, and is no doubt 
a device for increasing the area of the auricular walls 
for the purpose of excretion by means of the pericardial 
gland. This is confined to the surface of the auricles in 
Pecten, and gives the brown colour. It will be described 
later with the other excretory organs. 
The auriculo-ventricular openings are guarded by a 
series of circular fibres which function as_ valves 
and prevent the blood being forced back into the 
auricles. The auricles are also connected with each 
other by a broad transverse branch ventrally resting 
