THE LAMELLIBRANCHIA 



223 



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 Area ; traversed by the rectum in the great majority of 

 Lamellibranchia (Fig. 231, v) ; and finally ventrad of the rectum 

 in Malletia, Ostraea (except 0. cochlear), Mulleria, 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 Area 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 Pectvnculus. In these 

 diverse but relatively primitive forms (and also 

 in Pecten and some other types), the auricle of 

 either side is connected only with the anterior }[ 1 ' vfl V le '> 1V > aortu - 



M i i i V AIter * Oll.^ 



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 



FIO. 203. 

 Heart of Ostraea 



