33 



of muscle fibres (fig. 45, Br. in.') underlying the epithe- 

 lium but separated from it by connective tissue ; these 

 run, like the gill filaments, at right angles to the axis. 

 More remote from the surface there is a somewhat 

 scattered layer of fibres running in the direction of the 

 gill axis. Internal to these again and separated 

 from them by more connective tissue is another layer of 

 fibres running in the same direction as the first described 

 (fig. 45, Br. m."). 



In addition to the above there are two very important 

 compact bundles (fig. 45, Br. m.) which run longitudinally 

 along the gill axis. They are situated at the sides of the 

 axis just above the point at which the various filaments 

 separate from one another (Br. m.). In certain sections 

 of P. opercularis these muscles have shown a verv similar 

 striation to that of the pallial muscles. These last 

 muscles serve for contraction of the gills. 



The Cardiac Muscles.— The auricles and still more 



the ventricle are well supplied with muscle fibres. They 



extend around the heart, lying just internal to the wall 



and passing in various directions from the walls across 



the cavity dividing it up, so that it has almost the 



appearance of a sponge. These muscles are described 



with the heart. It is interesting to note here, however, 



that in the specimens fixed and sectioned, no traces of 



definite striation were found on these fibres, except in one 



case, resembling that of the radial, pallial and ctenidial 



muscles. 



FOOT. 



The Foot is a very small organ situated high up on 

 the anterior surface of the visceral mass (fig. 1, F.), 

 arising from the surface of the gonad close to the mouth. 

 In shape, it is roughly cj'lindrical with a sucker-like 

 termination (fig. 7, F. s.). This free end of the foot 



c 



