CH. VI] THE FRESH-WATER MUSSEL 173 



during protrusion; such alteration as there is is merely 

 increase of one part balanced by diminution of others. 

 The retraction of the foot after protrusion is not effected 

 by the " retractors " of the foot, but by the removal of the 

 blood aided by the contractions of the intrinsic muscles 

 which form so large a part of the ventral portion of the 

 foot and which are highly stretched during the period of 

 turgescence. 



Food and Digestion. The food of mussels is obtained 

 solely by aid of the ciliary currents already referred to. 

 The tentacles surrounding the inhalant siphon exercise 

 a certain power of selection, being extremely sensitive to 

 changes in the quality of the water. On the approach of 

 a substance of disagreeable flavour the adductors are 

 violently contracted and a strong current of water forcibly 

 discharged through both siphons washes the objection- 

 able matter several inches away. A fragment of tobacco 

 ash from my pipe falling through the water close to the 

 inhalant siphon of a specimen under observation was 

 treated in this way. 



The water is directed into the mouth along the 

 inverted groove formed by the ciliated labial palps of 

 which there is one pair upon each side. I am disposed 

 to think that nutritive particles are in some way caught 

 in mucous discharges and thus swept into the mouth, for 

 frequently in freshly captured specimens there is a line of 

 food-laden mucus between the palps and an accumulation 

 of the same substance at the mouth itself. The food 

 consists of various microscopic organisms and ddbris 

 of larger creatures. An examination of the contents of 



