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.
The first order, Copepoda, or oar-footed tribe, have a distinctly articulated body formed of movable rings, bristly swimming limbs; and the females carry their eggs in huge pouches suspended on each side of the posterior part of their bodies.
The Sapphirina fulgens is a beautiful example of the two-eyed tribe; its body is nearly oval, divided into nine distinct joints, and so flat that it is almost foliacious. The head has two brilliantly coloured eyes, with large cornea so connected with the shell that they look like spectacles. The two pairs of antennæ are silky, and the last pair of foot-jaws that cover the mouth are garnished with silky plumes. It has five pairs of swimming feet, and the tail ends in two little plates.
The Sapphirina is about a line and a half long, of a rich sapphire blue, and floats on the surface of the Mediterranean and tropical oceans. It shines with the most brilliant phosphorescent colours, passing from deep blue to a golden green, or splendid purple. The brilliant colouring is seated in the layer of cells that secrete the firm substance of the body. With a microscope the cells are seen to pass alternately from one colour to another. There is a little three-lobed body between the eyes connected with the central nervous system by a small nerve; it contains several corpuscules, which Professor Gegenbaur regards as the remains of the single eye of the larva which undergoes many transformations before it arrives at its adult form.
According to Professor Gegenbaur, the Sapphirina fulgens is a true Copepod and the Mediterranean Phyllosoma is a Decapod, although it has a lacunar blood system.
Some genera of the order Copepoda inhabit salt water, others fresh, as the Cyclops quadricornis ([fig. 150]), which abounds in the water with which London is supplied.
