150 



THE HARMONIES OF NATURE. 



Common Cockle. 



possess a more or less developed and variously-shaped muscular 

 foot, which they protrude at will from between the folds of 

 their mantle. By means of this organ many are able to dig 

 a hole or furrow in the sand, which enables them to baffle the 

 pursuit of many enemies, others to ad- 

 vance with a crawling movement or even 

 to make jumps along the sand. Thus 

 the common cockle stretches its foot (a) 

 out as far as possible, presses it against 

 the ground, springs up by suddenly con- 

 tracting it, and, by repeating the process 

 again and again, hops along at a pace one 

 would hardly expect to meet with in a 

 mollusc. In other genera, where the foot exists but in a small or 

 rudimentary form, the sudden opening and shutting of the valves 

 supplies its place as a means of locomotion. In this manner 

 the scallop, which inhabits deep places where it lies on a rocky 



or shelly bottom, swims or 

 flies through the water with 

 great rapidity, and the file 

 or rasp -mussel, a closely- 

 related genus, principally 

 occurring in the Indian 

 Ocean, glides so swiftly 

 along that even a light- 

 footed pursuer is hardly 

 able to catch it. In several 

 of the sedentary genera, 

 the foot, useless as an organ 

 of locomotion, is reduced 

 to the functions of spin- 

 ning a long lustrous and 

 silky fasciculus or bundle of 

 filaments called byssus or 

 beard, which serves to affix 

 the animal to any solid 

 body sunk in the sea. 

 Generally the connection is permanent, but some species, 

 among others the edible mussel, are able to detach the fila- 

 ment from the glandular pedicle situated at the inferior base 



Filament of a Byssus, highly magnified. 

 aaaa disc-like expansions. 



