In the Edriophthalmia are included the lower crustaceans that have no carapace, and whose thorax and abdomen are distinctly composed of articulated segments. The numerous legs are variously formed in the different genera for springing, walking, or swimming; and respiration is executed by certain portions of the extremities, modified for this purpose in their structure. To this order belong among others the saltatorial sandhoppers (Talitrus), which so frequently jump up before our feet when walking on the wet sea-sand; the ill-famed Cheluræ and Linnoriæ, whose devastations in submerged timber almost rival those of the ship-worm, and the parasitical Cyami, which gnaw deep holes into the skin of the whale. The sandhoppers are extremely frequent on the shores of the arctic seas, where they emulate the tropical ants in their speedy removal of decaying animal substances. Thus Captain Holböll relates that, having enclosed a piece of shark's flesh in a basket, and let it down to a depth of seventy-five fathoms, in the Greenland sea, he by this means caught within two hours six quarts of these little creatures, while a vast number still followed the basket as it was hauled up.
Chelura terebrans.
Limnoria lignorum.
Square facets of Scyllarus.
Hexagonal facets of Squilla.
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.