400 THE MUSSEL CHAP. 



mass, which passes below and in front into the foot 

 (Fig. lor, ft). Thus each valve of the shell is in contact 

 with the dorso-lateral region of the body of its own side 

 together with the corresponding mantle-lobe, and it is 

 from the epithelium covering these parts that the shell 

 is formed as a cuticular secretion. The whole space 

 between the two mantle-lobes, containing the gills, 

 visceral mass, and foot, is called the mantle-cavity. 



A single layer of epithelial cells, the deric epithelium 

 or epiclerm, covers the whole external surface i.e., the 

 body proper, both surfaces of the mantle, the gills, and 

 foot ; that of the gills and the inner surface of the mantle 

 is ciliated. Beneath the epiderm come connective and 

 muscular tissue, which occupy nearly the whole of the 

 interior of the body not taken up by the viscera, the 

 coelome being, as we shall see, much reduced. The 

 muscles are all unstriped, and are arranged in distinct 

 bands or sheets, many of them being very large and 

 conspicuous. The largest are the anterior and posterior 

 adductors (Figs. 101 and 103, a. ad, p. ad], great cylin- 

 drical muscles which pass transversely across the body 

 and are inserted at either end into the valves of the shell, 

 which are approximated by their contraction. Two 

 muscles of much smaller size pass from the foot to the 

 shell, which they serve to draw back : they are the 

 anterior and posterior retractors. A third extrinsic muscle 

 arises from the shell close to the anterior adductor, and 

 has its fibres spread fan- wise over the visceral mass, 

 acting as a protractor. The substance of the foot itself 

 consists of a complex mass of intrinsic muscles, which 

 aid in withdrawing the foot : its protrusion is largely 

 due to vascular turgescence. Lastly, all along the 

 border of the mantle is a row of delicate pallial muscles, 

 which, by their insertion into the shell, give rise to the 

 line already seen. 



