V. The Crustaceans ( Crustacea) have never less than ten feet; they have two compound eyes, and also antennae, which are generally four in number; their blood, which is white, is circulated by means of a muscular ventricle situated on the back: They respire by means of gills, and have no stigmata, or spiracles on the surface of the skin.
In the Articulate sub-kingdom, as in the ver tebrate, there may be traced one general plan of structure pervading all the classes, but with such variations in it as are, in each case, de manded by the particular exigencies of the individual to which it is applied ; but these variations are of such a nature, that a gradation of complexity or perfection may be followed through all the organic systems. With regard to locomotion, we commence with a class (the Cirripeds) as fixed and immoveable as the polypes and sponges of the Acrite sub-king dom; and afterwards trace a series of forms adapted first to slow and tortuous reptation ; next to swifter progression, as creeping, run ning, or leaping ; and, lastly, to a rapid flight through aerial space.
Generation, in like manner, is effected, in the lowest class, without the intercourse of separate individuals; afterwards by the reciprocal im pregnation of co-equal hermaphrodites, and, lastly, as in the vertebrate division, by indi viduals of distinct sexes.
The perfection of the nervous system results from the approximation of many separate gan glions into fewer masses of nervous matter. The organs of the senses also augment in number and complexity.
The Articulata pre sent, in the organs of the vital functions, as strongly marked differences as are met with in the vertebrate animals. With respect to the sanguiferous system, a gradation may be traced from a circulation in closed vessels to a diffused condition of the nutritious fluid ; and a corresponding pas sage from the articulata which respire by means of circumscribed branchia, to those in which indefinitely ramified trachea carry the air to all the parts of the body. The amount of respiration thus produced occasions the same effects here, as in the Vertebrate sub-kingdom, and the Insects thus constitute, as it were, the Birds of the Articulate division of animals.