another group, called _Copepoda_, become, when adult, so
degraded in structure as to have the appearance of mere worms, as
_Lerneocera_ and _Tracheliastes_, and become strangely unlike the
typical forms (crabs and lobsters) of their class.

Other animals of the class _Crustacea_, which animals form the order
_Cirripedia_ (barnacles and acorn-shells), bear such an external
resemblance to mollusks that they were actually classed by Cuvier in the
class _Mollusca_. In some of them--the Barnacles which commonly attach
themselves to the bottoms of ships--the head grows from above downwards
to a relatively enormous degree, forming the long stalk or "peduncle,"
at the lower end of which the small body with its limbs hangs suspended.

In another group, _Rhizocephala_, the form of the adult becomes yet more
strange. These creatures are parasitic on other crustacea. Having
attached themselves to the surface of the soft abdomen of the Hermit
crab, the head of the Rhizocephalon grows out into it as so many
root-like processes, from which condition the group has received its
name.

The numerous and long extinct group of _Trilobites_ also belongs to the
class _Crustacea_.

The next class, _Myriopoda_, consists of the hundred-legs (centipedes),
and thousand-legs (millipedes), which present us with some of the best
examples of creatures the bodies of which are composed of a longitudinal
series of similar segments. Allied to them is a very exceptional animal
found in Africa and New Zealand, and called _Peripatus_, the anatomy of
which presents many significant peculiarities.

The third class of Arthropods (_Arachnida_) consists of the scorpions
and spiders with their poor relations, the mites and tics, together with
the very peculiarly-shaped _Pycnogonida_ (which present us with a good
image of "no body"--being all legs and no body), and the singular
worm-like parasite _Linguatula_. Lastly, we come to the most
zoologically important and numerous of all the classes of
Arthropods--namely, to the "class" of insects--_Insecta_. Therein we
meet with the power of flight in its most perfect form--_i.e._, in the
Dragon-flies--and most of the species are aerial in their adult (or
_Imago_) condition. Some, however, are burrowers as, for example, the
mole-cricket--an insect which presents some curious analogies in
structure to the beast referred to in its name. Amongst insects may be
mentioned the most familiar of all, the House-fly (which bel