CHAPTER I 
CRUSTACEA—-GENERAL ORGANISATION 
THE Crustacea are almost exclusively aquatic animals, and they 
play a part in the waters of the world closely parallel to that 
which insects play on land. The majority are free-living, and 
gain their sustenance either as vegetable-feeders or by preying 
upon other animals, but a great number are scavengers, picking 
clean the carcasses and refuse that litter the ocean, just as 
maggots and other insects rid the land of its dead cumber. 
Similar to insects also is the great abundance of individuals 
which represent many of the species, especially in the colder 
seas, and the naturalist in the Arctic or Antarctic oceans 
has learnt to hang the carcasses of bears and seals over the side 
of the boat for a few days in order to have them picked 
absolutely clean by shoals of small Amphipods. It is said that 
these creatures, when crowded sufficiently, will even attack 
living fishes, and by sheer press of numbers impede their escape 
and devour them alive. Equally surprising are the shoals of 
nunute Copepods which may discolour the ocean for many miles, 
an appearance well known to fishermen, who take profitable toll 
of the fishes that follow in their wake. Despite this massing 
together we look in vain for any elaborate social economy, or for 
the development of complex instincts among Crustacea, such as 
excite our admiration in many insects, and though many a crab 
or lobster is sufficiently uncanny in appearance to suggest 
unearthly wisdom, he keeps his intelligence rigidly to himself, 
encased in the impenetrable reserve of his armour and vindicated 
by the most powerful, of pincers. It is chiefly in the variety 
of structure and in the multifarious phases of life-history that 
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