318 THE WORLD OF THE SEA. 
only with the oyster, but also in the shell of other bivalves, such 
as the mussels, the modiolas, and the pinne—indeed it is from this 
last mollusk that its name is derived, which literally signifies, 
the guardian of the pinne. Mr. Thompson examined eighteen 
mussels on the Irish coast, and they contained no less than four- 
teen of these fea-crabs. These were all females, but it is not 
uncommon to find in the same shell a male, with two or three 
females, and many young ones. 
The pea-crab is not selfish, for when it finds a good home it 
has an immediate desire to share its fortune, and soon the bivalve 
is favoured with a little colony of parasites. Thus again we are 
reminded of the harmony of Nature. Nothing is isolated; the 
order of life is as a chain, all the links fit into each other; not 
only is this the case in the economy of life, but in the inanimate 
world there is nothing independent. Every act of Nature has a 
cause which is not resident within itself, and bequeaths an effect 
which becomes the cause of a subsequent movement. 
With the crustaceans we terminate the invertebrate kingdom, 
and now we enter upon the highest of the divisions of the animal 
kingdom—the Vertebrata. Thus we have passed step by step 
from the simple to the compound—a word which we have used in 
three senses :—first, we applied the term to a moving or a fixed 
colony made up of a number of individuals so joined that they 
could not be said to be absolutely distinct. Again, in the case 
of the zoonites, where a number of distinct organisms were dis- 
covered forming the same animal, the creature was said to be 
compound ; and now for the third time we use the word to express 
a higher and more complicated structure. 
