THE SLEEP OF MOLLUSKS. 99 



the control of that community of impulses and purposes, and that 

 consensus of moral ideas and perceptions, which we call public 

 conscience. This influence is beginning to penetrate even the 

 darkest regions of Central Africa and to protect the unknown 

 barbaric tribes against the ravages of Arab slave traders and the 

 arbitrary authority of European adventurers. Each nation that 

 joins in this combined movement is doubtless seeking, first of all, 

 to further its own commercial and colonial interests; but it 

 suffices as an illustration of the prevailing spirit of the age that 

 the basis on which they profess to unite is the broad principle 

 of a common humanity. 



THE SLEEP OF MOLLUSKS. 



By CHARLES T. SIMPSON. 



IT is probable that the sleep or dormant period which mollusks 

 share in common with many other organic beings is brought 

 on not merely by the exigencies of climate, but that it is more or 

 less necessary in building up the wasting physical powers. All 

 organized beings seem to require rest in some form or other. If 

 plants, whether from the tropics or temperate regions, are kept in 

 hothouses, they will not grow the year round, and when forced 

 to do so soon become sickly or die outright. 



With the mollusks this sleep in many cases may be prolonged 

 indefinitely, often without the slightest apparent damage, and 

 under some conditions which seem really astonishing. 



In the sea the clams ( Venus and Mya) have rest periods, dur- 

 ing which they sink more deeply into the mud and retreat from 

 the fisherman ; the tritons,' murices, and ranellas form a shelly 

 growth and mark their seasons of repose by a thickening of the 

 aperture called a varix, which is sometimes guarded by spines or 

 knobs. The littorinas, which are amphibious, pass most of their 

 time on grass or sedges at the edge of the sea in the colder re- 

 gions and high aloft on mangrove or other trees in the tropics, 

 only occasionally going into the water to moisten themselves. 

 Tryon tells of some West Indian species which survived over a 

 year in his cabinet, and of others that lived for months in the dry 

 air of Philadelphia, though they exhibited but little activity ; and 

 the writer has kept specimens of the nearly allied Tectarius alive 

 in his collection for nearly two years. 



Most of the fresh-water species of mollusks pass a period of 

 hibernation in cold climates or testivation in the tropics, and 

 many of them are wonderfully tenacious of life when withdrawn 

 from the water. In June, 1850, a living pond mussel was sent to 

 Dr. Gray from Australia which had been kept out of water more 



