SOME ECONOMICS OF NATURE. 33 



veritable hydraulic engine. The effete water from the gills is ejected 

 with force from the funnel, and by the reaction of this jet d'eau upon 

 the surrounding medium, the animal is enabled to execute its aquatic 

 flights. Economy of a very rigid order is illustrated clearly enough 

 in octopod existence. The otherwise useless " breath " of the animal 

 becomes converted into a means of locomotion. 



A still closer parallel to the human chest-recoil, perchance, may 

 be found in the case of certain poor relations of the octopus. These 

 lower forms are the mussels, oysters, cockles, clams, and other bivalve 

 shell-fish which frequent our own and other coasts of the world. 

 Encased in its shell, a mussel or oyster, all headless as it is, and 

 possessing in its way a strictly "local habitation," in that it is a 

 fixture of the coast or sea-depth, presents us with the type of an ap- 

 parently vegetative life. But there is abundant activity illustrated 

 within the mussel or oyster-shell. There are millions of minute 

 living threads the cilia of the naturalist perpetually waving to 

 and fro as they crowd the surface of the gills. These cilia, acting 

 like so many microscopic brooms, draw in the currents of water 

 necessary for food and breathing, while the same incessant movement 

 which draws in the fresh water circulates it over the gills, and in turn 

 sweeps it out as waste material from the shell. The oyster implanted 

 in its bed, or the mussel attached by its " byssus " or " beard " to 

 the rock, exhibits a half-open condition of the shell as its normal 

 state. The animal lives as may be seen on looking at a tub of 

 oysters as they lie amid their native element with the shell un- 

 closed for purposes of nutrition and breathing. If, however, we tap 

 the living oyster or mussel ever so lightly, we find the shell to close 

 with a snap that renders the persuasion of the oyster-knife necessary 

 for its forcible unclosure. In such a case the animal's senses, warned 

 of possible danger by the tap on the shell, communicate to its mus- 

 cular system a nervous command, resulting in a movement which, 

 as regards the oyster, reminds one of nothing so forcibly as the cry 

 and action of " shutters up " in a Scotch university town when snow- 

 balling begins. 



The muscular system of these shell-fish is disposed in simple 

 fashion. Look at the inside of an oyster-shell, and note the thumb- 

 like impression you see occupying a nearly central position. This is 

 the mark of the " adductor " muscle of the oyster, or that which 

 draws the shells together. The secret of successful oyster-opening is 

 simply the knowledge, acquired by much practice, of hitting the 

 exact position of the " adductor " muscle, and of dividing its fibres 

 with the knife. The enormous power of this muscle to keep the 

 valves in apposition can be appreciated most readily, perhaps, by the 

 amateur " opener " of these bivalves. In the mussel there are two 

 such " adductors," one at either extremity of the shell, and we note 



D 



