The Crustacea breathe by gills, and are therefore suited to an aquatic
life. A few species, however, live in damp earth, or in places where the
air is damp (wood lice). Crustacea have two pairs of antennæ, and a
large number of appendages arranged in a characteristic way, but
differing very much in shape in the different groups; skin usually hard
and thick. Lobsters, crayfish, crabs, wood lice, and the small
sand-hoppers, water-fleas, etc., belong to the Crustacea. No Crustacean
is harmful agriculturally.
=Third Sub-Kingdom: VERMES= (WORMS).
Worms[3] are bilaterally symmetrical animals, in which the body is
enclosed in a “dermo-muscular tube.” Under the delicate epidermis there
is found a layer, which does not, as in the higher animals, consist
exclusively of dermis, but is partly composed of muscle-fibres, which
form a distinct coat internally. Worms are able to move by contracting
the various components of the dermo-muscular tube thus formed. In some
worms limbs assist in the movements, but in others this is not so; in
any case, however, the limbs play a relatively subordinate part. These
limbs, which are only present in the bristle-worms, are small, always
unjointed, foot-stumps, which bear bristles. Leeches and some other
worms possess suckers by which they can attach themselves, and move by
alternately contracting and extending their bodies.
Footnote 3:
In ordinary language one understands by “worms” elongated, cylindrical
animals; and several insect larvæ (_e.g._ “wireworms,” p. 103) have
the name applied to them. But to zoologists all “worm-shaped” animals
are not worms, nor have all worms an elongated cylindrical shape
(liver-fluke).
There are segmented and unsegmented worms (thread worms, liver-flukes);
the degree of segmentation is also very various. In many tapeworms each
joint may be regarded as an individual animal. The joints of other
segmented worms (earthworms) do not become detached, like those of
tapeworms, for the purpose of reproduction. The nervous system is absent
in the lowest worms; in the higher worms ganglia always constitute its
central parts, and in annelids these are arranged in pairs on the
ventral side of the body, and united by nerve fibres into a ventral
cord, which is connected in front with a nerve ring surrounding the gut
and thickened above into cerebral ganglia. In many worms (_e.g._
tapeworms and flukes) the nervous system is of much simpler structure.
The sub-kingdom of worms contains a very large number of forms. It is
only necessary to mention here representatives of three classes:
Segmented worms (_Annelida_), round worms (_Nematelminthes_), and flat
worms (_Platyelminthes_).