How are they distinguished from the Insects?--Barnacles and Acorn-shells.--Siphonostomata.--Entomostraca.--King-Crab. --Edriophthalmia.--Sandhoppers.--Thoracostraca.--Compound Eye of the higher Crustaceans.--Respiratory Apparatus of the Decapods.--Digestive Organs.--Chelæ or Pincers.--Distribution of Crabs.--Land Crabs.--The Calling Crab.--Modifications of the Legs in different species.--The Pinna and Pinnotheres.--Hermit Crabs.--The Lobster.--The Cocoa-nut Crab.--The Shrimp.--Moulting Process.--Metamorphoses of Crabs.--Victims and Enemies of the Crustaceans.--Their Fecundity.--Marine Spiders and Insects.
[Illustration: Development of Balanus balanoides.--(Acorn-shell.)
A. Earliest form. B. Larva after second moult. C. Side view of the same. D. Stage immediately preceding the loss of activity. _a._ Stomach. _b._ Nucleus of future attachment.]
The Crustaceans were included by Linnæus among his insects, but their internal structure presents such numerous and important differences that modern naturalists have raised them to the dignity of a separate class. They have indeed, in common with the insects, an articulated body, generally cased with hard materials; they are like them provided with jointed legs, with antennæ or feelers, and their organs of mastication are similarly formed; but insects breathe atmospheric air through lateral pores or tracheæ, while the crustaceans, being either aquatic animals or constantly frequenting very damp places, have a branchial or a tegumentary respiration. The perfect insect undergoes no further change; the crustacean, on the contrary, increases in size with every successive year. The higher crustacean possesses a heart, which propels the blood, after it has been aërated in the gills, to every part of the body; in the insect the circulation of the blood is by no means so highly organised. On the other hand many of the insects are far superior in point of intelligence to even the best endowed crustaceans, for here we find no parental care, no mutual affection, no joint labours for the welfare of a large community, no traces of an amiable disposition, but frequent outbursts of an irascible and sanguinary temper. Though the whole of the Crustacea are formed after one and the same general type, and the same fundamental idea may be traced throughout all their tribes, yet the rings of which their body is composed, and the limbs or appendages attached to these segments, undergo such extensive modifications of structure in the various orders into which the class has been divided that even the eye of science has with difficulty made out the true nature of many of their lowest forms. Who, for instance, judging from outward appearances alone, would suppose that the Barnacles and Acorn-shells which he sees riveted to the rock or to a piece of floating timber were relations of the crab or lobster; but a view of their early forms at once points out their real character, for then they appear as active little animals possessing three pairs of legs and a pair of compound eyes, and having the body covered with an expanded shield like that of many of the lower crustaceans. After going through a series of metamorphoses, these larvæ, tired of a roaming life, attach themselves by their head, a portion of which becomes excessively elongated into the "peduncle" of the Barnacles, whilst in the Balani or acorn-shells it expands into a broad disk of adhesion. The multivalve shell is gradually formed, the eyes are cast away as being no longer needed, and the now useless feet are replaced by six pairs of extremely useful _cirrhi_, long, slender, many-jointed, tendril-like appendages fringed with delicate filaments and covered with vibratile cilia. These cirrhi, which resemble a plume of purple feathers, and from whose peculiar character the name of the group, Cirrhipoda, is derived, are constantly in motion as long as they are bathed in water, projecting outwards and expanding into an oval concave net, then retracting inwards, and closing upon whatever may have come within their reach. They are so judiciously placed that any small animal which becomes entangled within them can rarely escape, and is at once conveyed to the mouth. The currents produced in the water by their perpetual activity serve also to aërate the blood, so that these delicate organs act both as gills and as prehensile arms. In spite of their sessile condition, the Cirrhipeds have not been left without protection against hostile attacks, for at the approach of danger they shrink within their shell, and close its orifice against a host of hungry intruders.