HORNELL ANATOMY OF PLACUNA 75 



the rectum, and in aspect its apex is turned slightly towards the left side (V. Plate II., 

 fig. 4). 



Its attachment to the visceral mass is by a narrow strip of tissue extending 

 upwards from the closely approximated entrances of the auricles to the emergence 

 of the aorta (Ao.). The ventricle is, therefore, not traversed by the rectum, nor even 

 closely approximated thereto ; indeed no intimate relation exists between them. 



In shape the ventricle is roughly of sub-globular form, with the apex pointing 

 posteriorly and slightly to the left Its greatest length in the largest of the preserved 

 adults examined was not more than five millimetres, the greatest breadth four mm. The 

 walls are muscular and extremely spongy, allowing thereby of great distension ; the 

 cavity is crossed by numerous muscular trabecula (Plate V., fig. 28). 



Two tubular auricles, one from each side (Au.r. and Au.l., figs. 16 and 17), enter 

 the ventricle at the ventral end of its attachment to the visceral mass ; the auriculo- 

 ventricular apertures ( V.a.v.) are placed close to one another, but each is quite separate. 

 Each is guarded by a clearly-defined mitral valve, consisting of two semi-lunar 

 membranes effectually preventing reflux of blood to the auricles. A single aorta, the 

 anterior, is given off from the ventricle a little way dorsal to the auriculo-ventricular 

 apertures, its opening provided with a single flap-shaped valve of simple structure 

 ( V.ao.). 



The auricles are paired and tubular ; they differ considerably in calibre another 

 instance of the far-reaching asymmetry of this mollusc, the left one being nearly double 

 the diameter of that on the right. They approach the ventricle horizontally from the 

 anterior aspect, and appear as two tubular arms embracing the base of the visceral 

 mass. They lie on the right and left sides, just beneath the thin tissue of the mantle 

 and midway between the dorsal aspect of the adductor muscle and the ventral portion 

 of the liver. Anteriorly each is continuous with a large vessel carrying blood from the 

 anterior apex of the ctenidium of its own side ; indeed the auricles appear as the 

 terminal dilated portions of these vessels, the common efferent branchial trunks 

 (C.br.efi). In this lack of specialisation the form of the auricles in Placuna is 

 particularly noteworthy. 



In common with the Pectinaceae and Ostrace?e, the auricles themselves independ- 

 ently intercommunicate by a short transverse channel (Au.ch., fig. 16), but Placuna 

 is remarkable in that this connecting passage is particularly wide, and situated relatively 

 so far distant from the ventricular end of the auricles as to be scarcely recognisable as 

 an inter-auricular channel. This abnormal situation is obviously consequent upon the 

 peculiarly elongated form assumed by the auricles. In one abnormal adult individual 

 obtained from Tuticorin (S. India) only one auricle, the left, opened directly into the 

 ventricle (Plate III., fig. 18). The right was connected in no way with the ventricle; 

 it was continuous posteriorly with the rectal sinus, and anteriorly with the right 

 efferent branchial trunk. The transverse auricular canal appeared very wide, so 



