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GENERAL INTRODUCTION
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sharply divided from the arms which always number ve,
though they may divide' and subdivide considerably, and
without the groove WhiCh always runs along the underside
of the arms in the starsh; the sea urchins (Echinoidea)
which are all globular, heart—shaped or disc—.shaped, with
a
rm skeleton or shell covered with spines and usually
with denite rows of tube—feet passing through ;, the sea
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cucumbers, sea gherkins or trepang (Holothuroidea) with
elongated,’ sausage-shaped bodies traversed generally ’by’
ve 'rows of tube-feet;
and nally the feàther-stars and
sea…—1ilies (CrinOidea) usually attached and With a central
disc withattachinents below, and above a series of ve,
often branching, arms whichwave about in the water.
The ARTHROPODA include the largest number— Of species
of any group in the animal kingdom.
Like the An_nelid
worms they have segmented bodies, but attached to all,
or many, of the segments are the jointed limbs—which give
the. group their name.
Of the four great subdivisions,
three“——the
Insect,
Spider and Centipede
almost
composed of land animals, as the
fourth, called: the Crùstacea,‘ are marine, One of the
examples of the latter found on land being the common
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woodlousé.
There is no spaceto go into the'many sub—
divisions Of the Crustacea, so all-embracinga group that-it
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includesthe tiny Water eas, the minute Copepods which
drift about in countlesS millions in the surface waters, the
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bamacles which cover rocks and the'bottoms of Ships, the
sand—hoppers
the shore, the
the
true
shrimps.and prawns, the lobster and craysh, the her-mit
crabs and the many kinds of true crabs.
The greatest
difference between the simplèr andniore complex types is
that in the latter the limbs, instead of being very much
of the/body to the other, have become
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specialized into a number of feeding limbs near the mouth,