v.] THE FRESH-WATER MUSSEL. 317 



b. The month; a wide aperture, lying immediately 

 behind and below the anterior adductor muscle. 

 It is encircled by the labial palps. 



€. Examine the thickened posterior border of the 

 pallial lobe. It is prolonged backwards for a 

 short distance as a pigmented tentacle-bearing lip 

 — the wall of the ventral siphon. Compare the 

 living animal, as it lies with its anterior end buried 

 in the sand, and note — 



a. The projection of the ventral siphon beyond the 

 free edges of the valves. 



/3. The close apposition of the thickened ventral 

 margins of the pallial lobes. 



y. The sipho7ial tentacles. A series of small con- 

 tractile finger-like lobes, the larger of which 

 project freely across the siphonal passage in 

 the path of the insetting current. 



Touch one of them with a bristle and note 

 the immediate closure of the siphonal aperture 

 ■ — the tentacles are highly sensitive. Examine 

 one of the longer ones under a lens ; its base is 

 swollen and it terminates in a slightly expanded 

 and flattened extremity. -^ 



S. Examine the relations of the gill laminae. The 

 outer one is confluent with the pallial lobe 

 externally and with the inner lamina internally, 

 for its whole length: the inner one is confluent 

 with the body wall for a short distance in front 

 and with its fellow of the opposite side behind ; 

 there is a spacious cleft between it and the body 

 wall. 



