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of very fine threads or filaments. These are hollow tubes 
and blood from the heart flows down inside them. Hach 
filament is covered by an immense number of very small 
hairs or celia, and as long as the mussel is alive these are 
waving backwards and forwards and causing a current 
of water to be sucked into the shell. This current passes 
over the gills towards the mouth. The food is strained 
by the gills, caught up by the lps, and taken into the 
mouth. At the same time the oxygen in the water passes 
through the filaments of the gills into the blood and the 
carbonic acid of the blood passes out into the water. 
Diagram of the structure of a mussel. Natural size. 
Breeding of the mussel.—Mussels are 
male and female. It is impossible to tell the sex except 
by examining the animal microscopically. The eggs and 
milt are contained in the mantle. Spawning takes place 
in the spring, and immediately on spawning the fish 
become thin and in poor condition because the eggs and 
milt have been expelled from the body. 
The little animal which hatches out from the egg is 
not at first like a mussel, and it swims about in the water. 
I 
