V.J THE FRESH-WATER MUSSEL. 317 



b. The mouth; a wide aperture, lying immediately 

 behind and below the anterior adductor muscle. 

 It is encircled by the labial palps. 



c. 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. 



/?. The close apposition of the thickened ventral 

 margins of the pallial lobes. 



y. The siphonal 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 gills. The outer 

 one is confluent with the pallial lobe externally 

 and with the inner gill 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. 



