148 Transactions of the Royal Society of Victoria. 



IY.--_FooT. 



The foot is a thick muscular brownish tongue-shaped body, 

 ventrall}^ situated, and its tip directed anteriorly. From the 

 posterior end, which is comparatively uncoloured, the byssus 

 for attachment is given off. By virtue of its secreting this 

 byssus, the foot is the fixing organ of the mussel, but the 

 free portion of the foot is capable of great expansion and 

 contraction, and is really a very active member. When the 

 valves gape a little it can protrude itself beyond the mouth 

 and outside the shell, or it can turn itself round and project 

 behind, or when the shell is firmly closed it may protrude on 

 the ventral surface. The foot is richly ciliated, there is a 

 slight notch at the free end, making the tip slightly bifid. 



If the free portion of the foot is detached and laid in water 

 sufiicient to cover it, movement will take place. The move- 

 ment is of two kinds — translatory and rotatory — the former 

 being the normal one. The direction of translation is straight 

 forward and away from the cut surface. The tip always led 

 the way, and it might sometimes diverge a little to the right 

 or left, but the general trend was a direct straight line. 

 The direction of rotation, with the dorsal surface uppermost, 

 was right-handed. The rate of rotation was, a complete 

 round in 6 hours 47 minutes. 



The rate of translation was fairly tested in a specimen, 

 with dorsal surface uppermost, which moved 6 inches in 

 5 hours 55 minutes, or at the average rate of 1 inch per 

 hour. With such a slow rate of movement, it is, of course, 

 impossible to say exactly when movement ceases. Accord- 

 ingly I have taken the safe plan of giving duration up to a 

 time after which a little movement was known to occur. A 

 specimen was thus known to retain its power of movement for 

 at least 73 hours, or about 3 days. 



Thus the wonderful result is arrived at, that in the 

 common sea mussel, which has been known and studied for 

 so long, there is a latent power of independent movement in 

 detached parts, which has hitherto escaped notice. 



It is one of the marvellous surprises of Natural History to 

 see the seeming biological paradox of parts when attached 

 to the living body apparently inert, but when detached from 

 it, in active motion. The gliding gill and the rotating palp, 

 the moving mantle-lobe and the creeping foot, show what a 

 stock of vital energy must be stored up in the soft-bodied 

 mollusc imprisoned within the walls of its shell. 



