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After a time it grows a shell and settles down as a fully 
formed little mussel. 
Sperm. 
Various stages in the growth of the mussel. (All magnified). 
The Development of the Flounder .— 
Eggs of the flounder will be examined with the micro- 
scope every afternoon. On the second day of the class a 
number of eggs will be taken from several female fish, and 
these will be fertilised by milt from male fish. A sample 
of this batch of eggs will be examined every afternoon, 
and the little fishes studied as they form inside the egg- 
shell. These fish will hatch out in from seven to ten days, 
according to the temperature of the sea-water. 
FourtH Day. 
The Tow-net and Plankton. 
Although the sea-water may look perfectly clear, yet 
it nearly always contains great quantities of very small 
animals and plants. These can be caught by means of a 
tow-net, which is a small net made of fine silk, or muslin, 
