v.] THE FRESH-WATER MUSSEL. 307 



a projection, which answers to the under surface of the 

 anterior adductor muscle, and the superior attachment of 

 the foot. On each side of the mouth are two triangular 

 flaps with free pointed ends — the labial palpi — and behind 

 these, on each side, two broad plate-like organs, with ver- 

 tically striated outer surfaces, are visible. These are the 

 gills or branchice. In the dorsal region, the integument is 

 soft and smooth; on each side^ it is produced into large 

 folds, the lobes of the mantle or palliiwi, which closely 

 adhere to the inner surface of the valves of the shell, and 

 end, ventrally, in the thickened muscular and glandular 

 margins already mentioned. They pass into one another 

 in front of the mouth; at the sides, they are united with 

 the dorsal edges of the outer gill-plates ; and, behind, 

 they extend upwards and on to the dorsal face of the 

 body, finally passing into one another above and in front 

 of the anus, which is small, tubular, prominent and 

 median. Thus the anus is inclosed in a part of the cavity 

 bounded by the two mantle-lobes which lies above the gills, 

 it is relatively small and shallow, and is termed the supra- 

 branchial or cloacal cha?nber; while the gills, the foot, and 

 the palps, hang down into the relatively large infra-branchial 

 chamber which occupies the space between the mantle- 

 lobes for the rest of their extent. During life the posterior 

 margins of the mantle lobes are prolonged for a short dis- 

 tance behind the free edge of the valves, and they come 

 into apposition at the point of attachment of the gills 

 in such a way as to give rise to two tubes or siphons — a 

 dorsal snpra-branchial one which receives the anus, and a 

 ventral one communicating with the infra-branchial chamber 

 alone. The dorsal siphon is the channel through which 

 the exhalent currents pass; the ventral, that for the inhalent 

 currents. 



20 — 2 



