Our ferocious Crustacean quietly continued its repast, when its companion seized it exactly as it had seized its prey, broke and tore it in the same fashion, penetrating to its middle, and tearing out its entrails in the same savage manner. In the mean time the victim, singularly enough, did not disturb itself for an instant, but continued to eat the first crab bit by bit, until it was itself entirely torn to pieces by its own executioner—a remarkable instance at once of insensibility to pain and of cruel infliction under the lex talionis. To eat and to be eaten seems to be one of the great laws of Nature.
Though essentially carnivorous, the Crustaceans sometimes eat marine vegetables. Many even seem to prefer fruit to animal food. Such is the robber-crab (Birgus latro) of the Polynesian Isles, which feeds almost exclusively on the cocoa-nut. This crab has thick and strong claws; the others are comparatively slender and weak. At first glance it seems impossible that it could penetrate a thick cocoa-nut surrounded by a thick bed of fibre and protected by its strong shell; but M. Liesk has often seen the operation. The crab begins by tearing off the fibre at the extremity where the fruit is, always choosing the right end. When this is removed, it strikes it with its great claws until it has made an opening; then, by the aid of its slender claws, and by turning itself round, it extracts the whole substance of the nut.
The Crustaceans have eyes of two kinds—simple and compound: the first are sessile and immovable, and very convex; the other borne on a short calcareous stem or peduncle, and formed of a number of small eyes symmetrically agglomerated—the reunion of all the microscopic cornea of a composite eye, resembling in shape a cap formed of facets. It is said, for instance, that the eye of the lobster consists of 2500 of these little facets. The simple eyes are myopus, or short-sighted—the compound eyes for more distant but perfect sight. They appear to have a strong sense of smell. Many of them cannot swim, but walk with more or less facility at the bottom of the water. It is said, for instance, that the cavalier of the Syrian coast, Oxypoda cursor (Fabricius), is named from the rapidity with which it traverses great distances.
Many systems have been proposed by different writers for the arrangement of the Crustacea. That proposed by Mr. Milne Edwards recommends itself, being founded on anatomical examination and actual experiment made by himself and M. Audouin. He divides them into two great divisions: I. Those in which the mouth is furnished with a certain number of organs adapted for the prehension or division of food. II. Those in which the mouth is surrounded by ambulatory extremities, the bases of which perform the part of jaws. The first includes the Maxilosa or Mandibulata, again divided into Decapoda, having branchiæ attached to the sides of the thorax, and enclosed in special cavities. The Decapoda are divided into: 1. Brachyura, namely, the Crabs. Cancer, Porlunus, Grapsus, Ocypode, and Doippe, belong to this group. 2. Anomoura, including Droma, Pagurus, Porcellana, and Hippa. 3. Macroura, including the Lobsters, Astacus, Palæmon, the Craw-fish, Palinurus.
Stomatopoda, with external branchiæ, sometimes rudimentary, sometimes none. Thoracic extremities prehensile, or for swimming generally, six or eight pairs. This division includes Mysids, Phyllosoma, Squilla, &c.
The Cirripedia, or barnacles, are a very important division of Crustacea; they are found in all seas, and attach themselves to almost every object in the sea; from the immovable rock to the moving animal; from the little Tunicata to the great turtle, or the whale.
The goose barnacles, Anatifa, have a flexible peduncle. The Balanoidea, or sea acorns, like oysters, are rooted to the spot on which they rest in their infant days; without the power, like the goose barnacle, of swaying to and fro like a pendulum, be their resting-place what it may.
One of the most remarkable animals of this class of Crustacea is the Limulus Moluccanus—the Molucca crab. They are distinguished by a long serrated spine, which looks most formidable. They are in great request in the markets of Java. Linnæus thought that the fossil trilobites were closely allied to the Limulus. Latreille, on the contrary, classed them near the mollusc, chiton. The tail of Limulus so strikingly resembles that of many Trilobites, that the most common observers may perceive an affinity.
Crabs and Craw-fish.
Crabs and lobsters may be regarded as the chiefs or lords of the Crustacean tribes. The crabs have very large claws and smooth backs; the last have small claws and the back covered with spines. Tiberius Cæsar had the face of a poor fisherman scratched by the rugged shell of a craw-fish.