HORNELL— ANATOMY OF PLACUNA 55 



a false appearance of a well-developed periostracum. Thorough washing, however, 

 reveals the mature shell to have no superficial horny layer, and to consist of two 

 layers only, the outer sub-translucent and with a micaceous cleavage, the inner 

 transparent in younger specimens and sub-translucent with a marked pearly white 

 lustre in older ones. 



That a delicate periostracum is actually secreted is shown distinctly in sections 

 through the mantle edge, where, as in Plate V., fig. 32, a delicate horny secretion 

 (periost.) is seen in process of formation by cells situated at the base of the periostracal 

 groove exterior to the true pallial margin {mg.<p.). 



GENEEAL ORGANISATION. 



In proceeding to a dissection of Placuna it is preferable to begin by removal of 

 the left valve, as the visceral mass and the majority of the organs are fused with or 

 sunk in the right mantle, whereas the left has retained very largely its primitive 

 character of a thin membranous fold serving as a loose envelope or cloak to the 

 remainder of the body. 



When the left valve is removed, this left mantle is seen to have lined the inner 

 surface of the valve except at a sub-central spot where one end of the large adductor 

 muscle (add.) pierces the mantle to find insertion on the inner surface of the shell, and 

 at the few small scars made by the insertion of the pallial muscles. 



The mantle edge is seen to be free at all points save dorsally along the hinge line, 

 where the two mantle folds meet and fuse. This dorsal union is complicated by the 

 intrusion of two enormous cardinal teeth (c.t.) with their ligaments, which entails this 

 dorsal region being split up into three portions — the central, bounded on either side by 

 a cardinal tooth, being conical in outline, while the two lateral are broadly spathulate, 

 and wider on the dorsal or free edge than at the base where they merge into the rest of 

 the mantle (Plate III., fig. 19). 



The dorsal edge of these lateral lobes in its turn is further subdivided into minute 

 spathulate processes, sometimes slightly bifid at their extremities, which alternate with 

 the minute hinge teeth found on either side of the cardinals. 



A small portion of the gonad penetrates these same lobes. 



Removal of the unattached portions of the left mantle exposes the visceral mass, 

 foot, labial palps and gills (Plate II., fig. 4). The first three lie on the side dorsal to 

 the adductor, while the gills beginning a little way above the anterior aspect of this 

 muscle pass first forwards and downwards, and then posteriorly along its margin in such 

 a way as to bound it on two sides. The visceral mass occupies the space between the 

 hinge and the adductor muscle. Broad and tumid ventrally, it narrows somewhat 

 dorsally, giving off" on that aspect two thin lobes to the anterior and posterior pallial 

 hinge flaps. 



