THE LAMELLIBRANCHIA eee} 
auricles are altogether asymmetrical. The ventricle lies com- 
pletely free in the pericardial cavity ; it is, however, fused to the 
dorsal wall of the pericardium throughout its length in Pliodon, 
and for a part of its length in Pandora. The walls of the 
ventricle are always very muscular, and contain free and inter- 
laced bundles or muscle fibres. The situation of the ventricle 
varies very much, even in tolerably closely related species: it 
is dorsad of the rectum in Nucula (Fig. 204), the Anomiidae, 
and Arca; traversed by the rectum in the great majority of 
Lamellibranchia (Fig. 231, v); and finally ventrad of the rectum 
in WMalletia, Ostraea (except O. cochlear), Miilleria, and Teredo 
(Fig. 195). The ventral position of the ventricle, in species 
remotely allied to one another, is a phenomenon of convergence 
due to the shifting of the base of the gill away from the primitive 
position of the heart. It should be observed that the transi- 
tion to the ventral position is to be seen in Pinna, Perna, and 
Avicula: in the first-named the ventricle still forms a very slender 
ring above the intestine, but in the two last genera it is simply 
attached for its whole length to the ventral side of the intestine. 
In Nucula and Arca the ventricle appears to be formed of two 
symmetrical halves: it is really elongated transversely, and con- 
tracted in the middle of its length. In adult Lamellibranchs the 
ventricle may beat rather slowly—e.g. twenty 
times per minute in the oyster, six times per 
minute in Anodonta—but in the young of 
Ostraea the pulsations may be as many as one 
hundred per minute. 
The auricles communicate with the ventricle 
by a narrow slit on each side, the apertures 
being provided with muscular valves which 
prevent the reflux of blood from the ventricle. 
The auricles are thick and muscular only in 
the Nuculidae, Solenomyidae, Anomiidae, and 
in a lesser degree in Pectunculus. In these Fic. 203. 
diverse but relatively primitive forms (and also Heart of Ostraea edulis, 
in Pecten and some other types), the auricle of [tr Steere escels 
either side is connected only with the anterior (Ame eay Dye 
or basal extremity of the efferent branchial 
vessel, a disposition which is common to other groups of Mollusca and 
indicates the primitively posterior situation of the ctenidia. In this 
case the auricles are elongated (Fig. 204, XII) and their maximum 
diameter is close to the ventricle. In all other cases their walls 
are thin and moderately muscular, they enter into relation with 
the gills along a considerable extent of the efferent branchial vessel, 
and they are triangular in form (Fig. 234, aur), with the maximum 
diameter nearest the gill. When the ventricle contracts the 
