The class Crustacea contains, besides the lobster (and its near allies, hermit-crabs, prawns, shrimps, and cray-fish), all crabs, including those very quaint-looking animals (now so often seen in our living collections), the king-crabs (Limulus), and a variety of more or less strangely different forms such as the following:—
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.
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 aërial 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 belongs to the order Diptera), and Beetles of all kinds (which constitute the order Coleoptera), some of which latter are luminous, as is the well-known glow-worm, and the exotic beetles Pyrophorus. Another order (Orthoptera) is made up of the earwigs, cockroaches, crickets, grass-hoppers, and their allies the locusts, with Bamboo-insects and the curious walking-leaf (so-called from their resemblance to a Bamboo twig and a foliage leaf respectively), the praying mantis, and other curious kinds.
Bees and Ants, which belong to the order Hymenoptera, are, as every one knows, celebrated for their wonderfully complex instincts and community-life (which will occupy us later), and to the same order also belong the Ichneumon insects, which are provided with long appendages at the hinder ends of their bodies wherewith to pierce the bodies of animals in order to deposit their eggs within them, or to pierce the substance of plants, so producing "galls" which are structures of much interest from several points of view.
Butterflies and Moths form another order of insects called Lepidoptera, amongst which may be mentioned as (having to be referred to hereafter) the true butterflies (Papilio), and the hawkmoths (some of which in their flight so much resemble Humming-birds), the clear-wing moths, and those moths the grubs of which are known as "silk-worms," and certain moths of the genera Solenobia and Psyche.