AN A TO MT OF THE CLAM. 



243 



the sandy bottom of the water, when the mussel is plough- 



ing its way through the 

 sand with its tongue- 

 shaped foot, which is a 

 muscular organ attach- 

 ed to the visceral mass, 

 and is a modification 

 of the under lip of the 

 larval mollusk. In the 

 foot is an orifice for 

 the passage in and out 

 of water, but the spurt- 

 ing of water from the 

 clam's hole, observed 

 in walking over the 

 flats, is the stream eject- 

 ed from the siphon. 

 The inflowing currents 

 of water pass from the 

 inner end of the mus- 

 cular siphon below the 

 lenticular visceral mass 

 to the mouth, which is 

 situated at the anterior 

 end of the shell, oppo- 

 site the siphon. The 

 opening is simple, un- 

 armed, without lips, 

 and often difficult to 

 detect. On each side 

 of the mouth is a pair 

 of flat, narrow-pointed 

 appendages called pal- 

 pi. The digestive ca- 

 nal passes through a 

 dark rounded mass, 

 mostly consisting of 

 the liver, covered ex- 

 ternally by the ovarian 



Fig. 161. A, Mya arenarii with its siphons ex- 

 tended ; in its natural position in the mud head- 

 end downwards ; B, transverse section of the shell, 

 showing the position of the spring opening the 

 shell; M, adductor mn PC! e ; the ligament represent- 

 ed by dark mass ; C, section of TTnio, showing the 

 position of the spring to open the shell ; L, liga- 

 ment ; I), ideal transverse section of Unio; /, intes- 

 auricle ; , gills ; 



