As the lower crustaceans offer but few points of interest to the general reader, they required but a few words of notice; but the highest order of the class, the Thoracostraca, thus named from the carapace which covers their thorax, so that only the abdomen presents an annular structure, may justly claim a more ample description. The preceding orders had either sessile eyes or none at all; here the movable eyes are fixed on stalks and of a compound structure like those of the insects; each ocular globe consisting of a number of distinct parallel columns, every one of which is provided with its own crystalline lens, receives its separate impression of light, and is thus in itself a perfect eye. Approaches to this structure are seen in some of the lower crustaceans; but here the "ocelli," as these minute individual eyes have been designated, are very numerous. They are at once recognised, under even a low magnifying power, by the facetted appearance of the surface of the compound eye, the facets being either square (Scyllari, &c.) or more commonly hexagonal (Paguri, Squillæ, &c). The auditory apparatus is likewise highly developed; the sense of smell is known to be very acute; and the antennæ are delicate organs of touch. The Thoracostraca are subdivided into the small group of the Stomatopoda, whose branchiæ are external and the feet prehensile or formed for swimming, and the far more numerous and important Decapods, which are either long-tailed like the scyllarus or short-tailed like the crab. In these the branchiæ no longer float in the water, but are enclosed in two chambers, situated one at each side of the under surface of the broad shelly plate which covers the back of the animal. Each of these chambers is provided with two apertures, one in the front near the jaws, the other behind. [Illustration: Scyllarus equinoxialis.]