HORNELL— ANATOMY OF PLACUNA 59 



PALLIUM OE MANTLE. 



The general disposition of the mantle has already been noted. We have seen that 

 its borders are without sutures between themselves save for a short distance along the 

 hinge line, where the two pallial lobes unite over the extreme dorsal region of the body. 

 The margins of the lobes are otherwise absolutely free ; they show no trace of siphons, 

 neither are the branchiae connected laterally with the inner surface of the mantle 

 either by organic fusion as in Anodonta, or by ciliar suture as in Margaritifera 

 vulgaris. 



Three regions compose each pallial lobe — a central, a distal or muscular, and a 

 marginal, all comparable and closely related in structure and position with the similar 

 regions seen in the mantle of Margaritifera. 



The central pallial area extends from the mid-dorsal line to the irregular and 

 imperfect pallial line where are inserted the radiating pallial muscles. The junction 

 with the distal area is not demarcated clearly as in the case of Meleagrinidse. The 

 greater portion of the central pallial area is adherent to the visceral mass on both sides, 

 and a comparatively small part is free. 



In both cases the mantle is thin and transparent, except where in the right 

 mantle masses of reproductive tissue and portions of the alimentary canal penetrate 

 and ramify in its substance. 



Where free from the complication of adhesion to the visceral mass, or of 

 penetration by glandular and other organs, the tissue consists of two parallel sheets of 

 epithelium connected by an excessively fine network of connective tissue. 



The distal or muscular region of the mantle is transparent save where masses of 

 black pigment occur in the external superficial tissue. The whole of its external face is 

 covered with secretory epithelium similar in histological structure to that upon the 

 exterior of the central pallial area. Below this outer epithelium occur a few scattered 

 glandular cells of large size, and containing slightly refractive granules. 



Strongly ciliated epithelium lines the inner surface of this pallial region and 

 subserves the double purpose of assisting in maintaining a steady indraft of water into 

 the branchial chamber and in ridding the pallial surface of sediment brought by the 

 incoming water current. In M. vulgaris a distinct narrow marginal ciliated " path " 

 exists, along which pellets of sediment are propelled, to the exhalent region ; here 

 there is no such path— the whole ciliated surface acts this part. In a newly-opened 

 living individual tiny pellets of mud may be seen at several points on the general inner 

 surface being propelled posteriorly, to be swept from the body by the current of the 

 excurrent stream of water from the giUs. 



Histologically there are present the usual characteristics of this region as seen in 

 other Lamellibranchs — a filling of connective tissue fibres in which radiate fan-shaped 

 muscle bundles, the trunks and bundles of the pallial nerve plexus and a great network 



