HO W SEA-URCHINS GRO W. 97 
sea-urchin left nestling in the seaweed has to grow up 
to a large animal, sometimes as big as a pomegranate, 
and yet its body is tightly shut in within lime walls. 
Look again at the shell after it is stripped of its 
spines (Fig. 40), and you will see that it is made of 
more than a hundred separate plates. While the 
animal is living these plates are covered within and 
without by a slimy film, and this film passes also 
between each plate. Now as the animal grows it 
takes fresh lime from the sea-water into this film, 
and places it, atom by atom, evenly on the edges of 
the plates, and so the shell grows with the body with- 
out disturbing any part ; and if this does not give 
sufficient room it can also add some plates to the 
top of the shell at the end of each ray. 
So the sea-urchin lives and grows, wandering over 
the seaweed beds and grazing with his powerful jaws 
as a sheep grazes in a meadow. Though the shells 
of animals are sometimes found in his stomach they 
are not his proper food, for he is a vegetarian and 
might probably almost be said to chew the cud in 
his powerful jaws, which Aristotle called by the 
curious name of " lantern " from their peculiar shape. 
He has many powerful enemies, and his shell is 
often found in the stomachs of large fish and other 
sea-animals ; so that besides his strong box he has 
great need of his spines for protection, and he can 
give very sharp pricks with them from out of his 
hiding-places when he is interfered with. His spines, 
however, serve many other purposes. They guide 
him when he walks, they help him to burrow in the 
sand, they have even been seen passing seaweed and 
other objects over his body, and they help the little 
