SOME ECONOMICS OF NATURE. 677 



tion has to be subserved, and when the cuttle-fish desires to swim, it 

 propels itself through the water by aid of a veritable hydraulic en- 

 gine. The effete water from the gills is ejected with force from the 

 funnel, and by the reaction of this jet (Peau 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. In- 

 cased in its shell, a mussel or oyster, all headless as it is, and possess- 

 ing 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 apparently vegeta- 

 tive 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 breath- 

 ing, w r hile the same incessant movement which draws in the fresh 

 w r ater 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 mus- 

 sel 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 na- 

 tive element with the shell unclosed 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 per- 

 suasion 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 muscular 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 fash- 

 ion. 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 fibers 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 the impressions which these struct- 



