262 POPULAR HISTORY OF THE AQUARIUM. 



show that the species was not only thriving, but freely 

 breeding in their present position. I must confess, how- 

 ever, that I felt far from satisfied with the demonstration ; 

 for while the young seemed to be brought forth plentifully 

 and to grow freely, the old ones seemed to die off rather 

 fast ; and I cannot imagine why the poor creatures should 

 not be allowed to take an airing when so disposed. 



The movements of Periwinkles are so slow, and they 

 remain so long in a given spot, that seaweeds often take 

 root and grow on the outside of their cumbrous shells. 



They are very useful in a tank, their occupation being 

 that of scavengers. The seeds of marine plants held in 

 suspension in sea-water, are apt to fall and adhere to sur- 

 faces and begin to grow, and their accumulation tends not 

 only to obscure the glass so as to check our observa- 

 tions, but also their too great abundance tends to render 

 the water unhealthy. To check this overgrowth and pre- 

 vent the glass sides of the tank from becoming opaque, a 

 few Periwinkles are put in, and very soon put in motion 

 an apparatus well adapted for mowing down the minute turf 

 of Co)iferv(B. 



This apparatus consists of a rasp-tongue, set with curved 

 teeth. If we notice one of these animals feeding with his 



