Certain Crustaceans, of the group called _Ostracods_, have the hard outer coat of their body so peculiarly modified that they have quite the appearance of Lamellibranch Mollusks, and this resemblance is even more than skin deep, as we shall see later. Some of 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.