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run we find that the smallest animals in the sea live on 
the Diatoms. 
Thus— 
The Cod feeds largely on Crabs, 
The Crab _,, Rp} ,, Worms, 
The Worm ,, . ,, Diatoms. 
The Plaice feeds largely on Shellfish (mussels and 
cockles), 
Shellfish _,, ‘A , Diatoms. 
The Herring and Mackerel feed largely on Copepods, 
Copepods feed on Diatoms. 
So everything im the sea depends for its food on the 
Diatoms or on other plants living in the sea. The 
Diatoms and other plants feed on the carbonic acid in the 
sea water and on substances containing nitrogen which 
are washed down from the land into the sea. 
A number of pictures will be shown by the lantern on the 
last afternoon of the first week of the class. These pictures 
will be explained, and they will illustrate the work of the first 
week. 
SrxtH Day. 
The structure of Fishes. 
Skates and Rays and Dogfishes.— 
These fishes difter in many ways from the round fishes 
like the cod and haddock. Their bones are gristly, they 
breed in a different manner, and their gills are different. 
The structure of a skate—Its bramang 
nerves; the ear; the heart, stomach and intestine; the 
gills. 
