ANATOMY OF THE CLAM. 



223 



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 m 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. 155. A. Mya arenaria with its siphons extended ; 

 in its natural position in the mud head-end downwards. 

 B, transverse section of Unio, showing the position of the 

 spring opening the shell. M, adductor muscle ; the liga- 

 ment represented by dark mass. C, section of Mya.show ng 

 the position of the spring to open the shell ; L, ligiuiient. 

 D, transverse section of Unio (after Brooks) ; ab, visceral 

 mass ; o, auricles ; v, ventricle , ;', intrstinr ; t, glandular 

 part of kidney ; z, non-glandular part of kidney ; y, sinus 

 venosus ; iy, inner, eg, outer, gills ; m, mantle. 



