ARTICULATA.
In the great division of invertebrate animals called Arti culata the brain is in the form of a ring encircling the gullet. A double ganglion above the tube supplies the chief organs of sense. The ganglions below the tube are connected with two chords which extend along the ventral surface of the abdomen, and are in most species united at certain distances by double ganglions, which are connected with the nerves supplying the body segments and their appendages. The body presents a corresponding symmetrical form. The skeleton is external, and consists of articulated segments of a more or less annular form. The articulated limbs, in the species possessing them, have a like condition of the hard parts, in the form of a sheath which incloses the muscles. The jaws, when present, are lateral, and move from side to side.
The worm, the lobster, the scorpion, and the beetle, ex emplify this province.
The articulate division of the animal kingdom, most uni versally distributed and numerically abundant at the present day, is least perfectly represented amongst the relics of the former world. Their chitinous integuments, often hardened with earthy salts, are quite as capable of preservation as the shells of the Mollusca, and remains of them are met with in all aqueous deposits ; but that manifold, complex organization, which in the recent state fits them so admirably for generic and specific comparisons, is fatal to their entire preservation, and the fossil examples are often so fragmentary as to admit of little more than the determination of their class and family.
The most ancient fossiliferous rocks bear imprints which have been regarded as the tracks and burrows of marine worms. With these are found Crustacea of the lowest division, and of a group which is wholly extinct. A little later appear the Phyllopods, Copepods, and other existing orders of Entamos traca. Only a few obscure forms, doubtfully referred to the higher division, Mala,costraca, have been found in the carboni ferous and Permian systems. The secondary strata contain abundant remains of Isopods, and of lobsters and hermit-crabs. True crabs (Brachyura) abound in the oldest tertiaries. Air breathing insects and Arachnids existed even in the palmozoic age ; the " sombre shades" of the carboniferous forests were not " uncheered by the hum of insects ;" nor were the insects blind, like those which now inhabit the vast caverns of Ken tucky and Carniola. The Articulata which come latest are the Cirripedes, whose lowest family appears in the lias ; while the Balanichz are only found in the tertiaries.
The number of fossil Articulata catalogued and described forms but a very small proportion of those which have pro bably existed. Bronn enumerates 1551 fossil insects, 131 arachnids, 894 crustaceans, and 292 anellides. Darwin de scribes 69 fossil cirripedes, 12 of which are living species.