In the swimming Isopods, the five first pairs of tail-limbs are false feet, and are suspended under the tail. The gills, consisting of two great oval leaves, are fixed to them by a stalk; and are dragged through the water. This group is very numerous; many live among the sea-weeds on the coasts, others perforate submerged wood in all directions, and live in the winding galleries they have formed. The Limnoria lignorum is particularly destructive in the harbours on the British coasts, and in the locks of the canals. The tortuous holes it bores are from the fifteenth to the twentieth of an inch in diameter, and about two inches deep. The female Isopod is not more than a line or two in length, the male is a third less, and of a grey or greenish brown. These minute creatures bore their holes with their mandibles, which are so sharp and strong that they can penetrate the hardest wood, and appear to feed on it, from the quantity found in their stomachs. Their bodies are covered with pinnated hairs, their antennæ are short, and their posterior end or tail is rounded. Most of the genus Cymothea are parasitical; they can bend the sharp nail of the three first pairs of feet upon the preceding joint, so as to form hooks with which they fix themselves to the fishes on whose juices they feed. The Isopods bear a strong resemblance, an almost identity of structure, with the Trilobites, a jointed race of Crustaceans long extinct. Some of the Isopods roll themselves into a ball, as these most ancient inhabitants of the ocean were wont to do; whose large compound eyes are exactly like those of the Isopods; whence it was inferred by Dr. Buckland, that neither the constitution of the sea nor the light of the sun had changed for innumerable ages. The discovery of the Eozoön has proved that Nature has not varied during a period immeasurably prior even to that. _Entomostraca._ The Entomostraca form an immense group of the lower Crustacea, consisting of five orders. A vast number are just visible to the naked eye, and many are microscopic; they teem in every climate along the coasts, and in the deep blue oceans. The horny coat, enclosing the minute bodies of these animals, is often so transparent that their internal structure, and occasionally the process of the assimilation of the food, is distinctly seen by the aid of a microscope. Small as they are, their beauty is often very great; when transparent they sometimes radiate all the prismatic colours; when opaque, they are frequently of the most brilliant and varied hues, others shine with vivid phosphorescent light. The segments of their bodies are often very numerous, and similar to one another; but their appendages are very different. They form two distinct natural groups of the bristly-footed and gill-footed Crustacea. _Copepoda._