32 



The fibres are inserted at a eonsiderahle angle, and 

 from the shell, they pass first as a flat band and then, 

 becoming circular in section, across the dorsal surface of 

 the adductor, directly towards the base of the foot. This 

 brings the Retractor under the pericardium and the 

 digestive gland until it reaches the visceral mass, through 

 which it plunges, just at the junction of the digestive 

 gland and reproductive organ, lying enclosed in a fairly 

 definite blood space. Transverse sections through the 

 muscle close to the base of the foot show (fig. 47, B. g.) 

 that the muscle has taken a tube-like form, enclosing the 

 byssus gland. The muscle fibres pass around the gland, 

 which has the form of compressed pouches separated by 

 lamellae of connective tissue, and ultimately they become 

 lost in the tissue of the foot. 



The Intrinsic Muscles of the foot make up the bulk 

 of the tissue in this part of the body (fig. (S). They run 

 chiefly in two directions. There is a definite layer of 

 circular muscle fibres underlying the surface and 

 extending all round tlie foot, more internal still is a 

 series of longitudinal muscles running along the axis of 

 the foot. In addition there are many fibres diverging 

 radially from the centre, and also scattered fibres passing 

 in various directions. 



Other intrinsic muscles are to be found in the visceral 

 mass in the reproductive region. There is a layer of 

 transverse muscles passing round in the connective 

 tissue sheath which encloses the visceral mass, and con- 

 nected M'itli this sheath are scattered muscle bundles 

 running across from one side to the other and serving to 

 strengthen and form a support for the alveoli of the 

 gonad. 



The Ctenidial Muscles (fig. 45, Br. m., Br. m.\ 

 Bv. III.") are arranged as follows: — There is first a layer 



