400 THE MUSSEL CHAP. 



below and in front into the foot (Fig. 101, //). 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, contain- 

 ing the gills, visceral mass, and foot, is called the mantle- 

 cavity. 



A single layer of epithelial cells, the deric epithelium or 

 epiderm, 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 cilia- 

 ted. 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 ccelome 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 

 WbA posterior adductors (\gs. 101 and 103, a. ad, p. ad], great 

 cylindrical 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 muscle arises from the shell close to the 

 anterior adductor, and has its fibres spread fan-wise over the 

 visceral mass and 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. 



