172 THE FRESH-WATER MUSSEL [CH. VI 



a rough stone some three inches high. Normally, however, 

 they prefer to move along the level muddy bottom, in 

 which they plough furrows that may often be tracked for 

 several yards. 



The method by which the protrusion of the foot is 

 brought about has been the subject of much discussion 1 . 

 When fully distended this organ becomes far more trans- 

 parent in appearance and is obviously dilated by some 

 fluid substance within. It has been maintained by some 

 investigators that water-pores exist through which water 

 is taken in for the purpose of dilation. Fleishmann 2 

 and others have, however, satisfactorily established that 

 the supposed pores are either artificial injuries or the 

 apertures of mucous glands affording no communication 

 between the blood and the outer water. The entire 

 phenomenon is dependent upon the distribution of the 

 blood within the body. During rest the blood, which 

 forms fully half the weight of the body, is largely 

 contained in the sinuses of the mantle. When the foot 

 is required for use 3 an inrush of blood is permitted into 

 the pedal lacunar spaces, but egress is prevented by a 

 special sphincter muscle, known as " Keber's valve," 

 which closes the great vein leading from the foot to 

 the nephridia. The tissues of the foot thus become 

 swollen and turgid and relatively firm for looomotor 

 purposes. It is noteworthy that there is no actual in- 

 crease in the volume of the soft parts of the animal 



1 Cattle, Zool. Anz. vi. 1883. Griesbach, ibid. vii. 1884. 



2 Zeit. wiss. Zool. XLII. 1885. 



8 Willern, Mem. Gour. Acad. E. Sci. Belg. LVII. 



