64 THE BOOK OF NATURE STUDY 



prevent either the shell from slipping down over it or the foot 

 itself from being drawn back into the shell ; then with a convul- 

 sive heave the shell rides forward on the foot, and finally is pulled 

 a little downwards as the adductor muscles relax and the valves 

 gape apart. At the end of each " step/' although the valves 

 separate, the free edges of the mantle-folds remain in contact, 

 probably by contraction of the pallial muscles. There then 

 results a diminished pressure inside the shell, and the protruding 

 siphons are crushed together by the excess of outside pressure, 

 which perhaps thrusts the shell down into the mud. The forward 

 movement of the shell (i.e. the whole animal) is brought about 

 by the contraction of the so-called retractors of the foot. As 

 already stated, it is the " foot " which is fixed, the shell movable. 

 If by chance the animal gets into a " tight place " between the 

 stones or other obstructions it is able to crawl out backwards 

 by the use of the muscles which draw back the shell (protractors 

 of the "foot"). It is to be noted that locomotion is effected 

 by a series of steps, and not by the steady gliding movement 

 characteristic of slugs and snails. The rate of movement is very 

 slow about a mile a year. The withdrawal of the " foot " 

 is accomplished by the contraction of the numerous muscle fibres 

 in its ventral region, accompanied by the simultaneous release of 

 the valve which dams up the blood within ; it is possible that the 

 " retractors " may assist in pulling it back within the shell. The 

 furrows made by mussels are often plainly visible along the 

 bottom of still waters. The labial palps lie on the sides of the 

 foot in its anterior dorsal region ; in appearance they rather 

 resemble the gills, but are yellow rather than brown. There 

 are two of these organs on each side ; they are triangular in 

 shape, and those of one side are united dorsally so as to enclose 

 a narrow gutter open ventrally. The gutters lead into the 

 mouth just below the anterior adductor muscle. The labial 

 palps are richly ciliated, and appear to produce a special slime 

 for the purpose of catching the microscopic vegetable and animal 

 organisms contained in the water brought in through the in- 

 halant siphon, for the top of the inverted groove is usually occu- 

 pied by a band of slime heavily charged with organic matter. 

 The food is thus led into the mouth and driven along the ali- 



