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are so extremely abundant in the sea, that when they die 
their dead shells fall to the bottom, and form a layer of 
mud or ooze. 
The difference between an animal and a plant. 
Most animals have a very different appearance from 
plants, but in the case of diatoms, and other small 
creatures, it is quite impossible to tell by their appearance 
whether they are animals or plants. The real difference 
hes in the methods of feeding. 
An animal must live on food which is either alive, or 
has been alive. So animals feed on the living or dead 
bodies of other animals or plants. 
A plant lives on material which has nevet been alive 
—~on the carbonic acid and the substances containing 
nitrogen, which are in the air or are dissolved in the sea- 
water. 
Maenified about 70 dia. 
Noctiluca. 
N oetiluca.—tThis is a small animal which is 
always present in some parts of the Irish Sea during the 
summer. It is sometimes so abundant as to colour the 
water brown. It is simply a httle round blob of jelly-like 
material. It feeds on diatoms, and these will usually be 
