VII MUSCLES 385 



tinned ventrally into a keel-like visceral mass, which passes 

 below and in front into the foot (Fig. 93, //). 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 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. 93 and 95, 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 shell to the foot, which 

 they serve to draw back : they are the anterior and posterior 

 retractors of the foot. A third foot-muscle arises from the 

 shell close to the anterior adductor, and has its fibres spread 

 fan-wise over the visceral mass which it serves to compress, 

 thus forcing out the foot and acting as a protractor of that 

 organ. The substance of the foot itself consists of a 

 complex mass of fibres, the intrittsic muscles of the foot, 

 many of which also act as protractors. Lastly, all along 

 the border of the mantle is a row of delicate pallial musclesj 



Pbact. Zool. C C 



