NATURAL HISTORY. 65 
The common duck-weed will answer this pur- 
pose; but not so well as most other aquatic 
plants, as it is so rapidly decomposed in hot 
weather. 
Before changing the water, the polypi must 
be removed by a feather into a goblet of the 
same water, in order that the sides of the glass 
may be well cleansed from the dirt and spon- 
taneous confervee which adhere to it; for, if 
these are permitted to accumulate, the creatures 
will not thrive, although a due supply of food 
and fresh water be afforded them. 
Small larve, crustacea, or worms, must be 
supplied them, or in the absence of these small 
snails (Helix planata), or pieces of raw meat 
cut very small, and carefully dropped into the 
water over the place where they are situated, 
that it may fall within their reach. 
In cold weather they must not be exposed 
too near the window, as they are very tender, 
and become torpid. 
