SHELL AND MANTLE 



273 



the two ends, though at the extreme ends it turns upwards to 

 the hinge Hne. At the hind end each mantle edge is fused for some 

 distance with its fellow ; it then separates widely from it twice, 

 so as to form the figure of 8 already mentioned, and lies against 

 its fellow for the rest of its length. The upper opening is known as 

 the dorsal siphon, the lower as the ventral siphon. The lips of 

 the latter bear a fringe of small tentacles. The space enclosed 

 by the two mantle lobes is known as the mantle cavity. 



EXTERNAL FEATURES I LOCOMOTION AND FEEDING 



It will have been noticed that the shell and mantle of the mussel 



gen. 

 I. p. ; k.o. l-ml. 



a.ad. 



Fig. 200. — A swan mussel removed from its shell and lying on its right side with 

 the left mantle lobe and left gills turned back. A portion of the inner lamella 

 of the left inner gill has been cut away to show the openings of the kidney 

 and gonad. 



a.ad.. Anterior adductor muscle ; d.s., dorsal siphon ; /., foot ; gen., opening of the duct of the gonad ; 

 k.o., opening of the kidney ; l.i.g., left inner gill ; l.ml., left mantle lobe ; l.o.g., left outer gill ; l.p., labial 

 palps ; r.i.g., right inner gill ; r.ml., right mantle lobe ; r.ml'., thickened edge of the same ; r.o.g., right 

 outer gill ; v.s., ventral siphon. 



are bilaterally symmetrical. The same symmetry is found in all 

 the other organs of the body, both internal and external, except 

 for the slightly coiled gut. Above the attachment of the mantle, 

 at its lowest point near each end, may be seen on each side the 

 cut surface of the great adductor muscles, anterior and posterior, 

 which pass through the body from side to side and draw together 

 the valves of the shell. To the upper and inner sides of these lie 

 the anterior and posterior retractor muscles, which draw the 

 body forwards upon the foot when the latter has been thrust 

 out. Behind the lower end of each anterior adductor is a protractor 



