ANODONTA 57 



passing into the body just below and behind the anterior 

 adductor muscle. Posteriorly the foot is overlapped by the 

 anterior ends of a pair of long flat lamellate structures shaped 

 somewhat like pea-pods. These are the outer and inner gill- 

 plates. On turning them up the posterior end of the foot is 

 seen to be prolonged into an elongate muscular band which 

 may be traced backwards to near the hinder border of the 

 posterior adductor. There it bifurcates, and its right and left 

 branches are attached to the valves of the shell at the spots 

 which have been described as the impressions of the posterior 

 retractors of the foot. The whole of the dorsal and dorso- 

 lateral integument, excepting a prominent ridge along the 

 mid-dorsal line, is thin and transparent, and some of the 

 viscera can be clearly seen through its walls. In the mid- 

 dorsal region there is a considerable space, looking dark when 

 seen through the thin body-wall. This space is the peri- 

 cardium, and it is traversed by the posterior part of the gut, 

 the latter being wrapped round by the ventricle of the heart. 



There is no head, the fresh-water mussel, in common with 

 all bivalve or lamelli-branchiate molluscs, being destitute of any 

 definite prostomial region. The anterior adductor muscle, 

 which lies where the head should be, must not be regarded as 

 representing the head region, but rather as a special muscular 

 development of the anterior part of the mantle. The body of 

 the mussel, then, consists of a foot below, and a shapeless 

 dorsal region containing a large part of the viscera. This 

 latter region we may call the visceral hump. The wall of the 

 visceral hump is folded downwards on each side of the body to 

 form the extensive mantle-flaps, which secrete the bivalve shell. 



The anterior ends of the gill-plates are attached to the body- 

 wall in the bay formed by the curved line of attachment of the 

 mantle a little way below the umbonal region. Just in front of 

 and below the anterior ends of the gills is a pair of triangular 

 flaps lying below the attachment of the mantle. The apices of 

 the triangles look downwards and backwards, their bases run 

 downwards and forwards from the anterior ends of the gills to 

 the angle between the upper anterior edge of the foot and the an- 

 terior adductor muscle. These flaps are called the labial palps. 

 Their edges are continued anteriorly round the front edge of 

 the foot, and pass into two similar palps on the opposite side of 

 the body. The labial palps form the upper and lower borders 



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