72 THE MARINE AQUARIUM. 
as evidenced in the construction of the most humble 
creatures. The winkles accomplish for the marine-tank 
what the fresh-water snails do for the river-tank, they 
scrape confervoid growths from the glass, and so help to 
preserve the crystalline aspect of the tank. All the 
species of winkle are capable of domestication, Littorina 
littorea, the commmon sort, and £. littoralis, a pretty 
little fellow, with a gaily mottled hybernaculum. 
The Trochus tribe, better known as Tops, are also 
useful as cleaners, and in appearance are more stately and 
ornamental than the winkle, their cleanly marked conical 
shell attracting as much attention from strange eyes as 
the noble planorbis corneus does in the river-tank. Gene- 
rally speaking, univalves are more easily kept than bivalves; 
many of the latter are apt to die off, and cause some 
amount of putrescence before their demise is discovered. 
