75 



^1/. c. J, aud fig". 4'j), as in the iiiajority of lamcllihianclis. 

 The Avail of the rentricle 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 

 granular 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. Their 

 shape is roughly conical, the base being u})permost 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 dejjressions of the inner surface (tig. 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 



