CH. Vl] THE FRESH-WATER MUSSEL 171 



can be very plainly seen to be the case in the region of 

 the siphons. These remain projecting posteriorly but are 

 closed, and, at the moment of relaxation of the adductors 

 and consequent separation of the valves, are forcibly and 

 suddenly flattened from side to side. It is possible that 

 advantage is taken of this hydrostatic pressure outside 

 the animal in forcing the shell downward into the soft 

 mud. Immediately afterwards the mantle borders separate 

 along their entire length, water rushes in everywhere and 

 the siphons reopen. 



Prior to each fresh effort in a forward (or backward) 

 direction the mantle borders are again approximated, 

 nor is any effort made until their closure is complete. 

 These facts were ascertained by repeated observations 

 of a specimen lying partly on its side close against 

 the glass of an aquarium. The forward movements 

 described are brought about by the contractions of 

 the so-called retractor muscles of the foot : it is obvious 

 that "protractors of the shell" would be a more appro- 

 priate title. Progression, however, is not the only move- 

 ment of which the mussel is capable ; I have seen 

 specimens move backward out of a narrow cleft after vain 

 endeavours to force a passage, or in order to make a 

 second attempt to get round an awkward corner, and 

 this they can do just as successfully and forcibly as 

 forward locomotion. In this case the shell rides backward 

 over the foot, being drawn back by the so-called protractor 

 of the foot (retractor of the shell). There is considerably 

 more power of surmounting obstacles than might be ex- 

 pected : I have frequently seen large specimens get over 



