HABITS OF MARINE ANIMALS 153 
animal and vegetable life known as protozoa and 
protophyta, of which diatoms are the most important. 
Protozoa and protophyta each consist of but a single 
cell, and are among the lowest forms of life to be found 
in the sea. 
The seal feeds on the cod, the cod on the whiting, 
the whiting on the sprat, the sprat on the copepod 
(a minute form of crustacean always present in the sea), 
and the copepod on protozoa and diatoms. 
Again the ray devours the plaice, the plaice devours 
the worm, and the worm feeds on various low forms of 
marine life, which exist on diatoms, and so protozoa 
and diatoms are the ultimate food producers. 
How comes it that diatoms can meet the immense 
demands that must be made upon them, since each 
of these consist of but a single vegetable cell, not the 
size of a pin’s head? It has been calculated that a 
diatom, dividing in two, as it usually does five times in 
a day, would at the end of a month, if no destruction 
occurred, form a mass a million times as big as the sun. 
Thus we see that diatoms and other microscopic 
plants constitute the pastures of the sea upon which 
everything, including man, ultimately feeds. 
Now life in the sea may be conveniently divided 
into three great divisions. Firstly, fishes and some 
crustaceans which roam from end to end of the ocean ; 
secondly, the life which lives on the bottom, such as the 
sea-weeds, sponges, shell-fishes, star-fishes, and the 
fish that are of sedentary habits; and, lastly, the life 
