Insecta, differ from them, in regard to external parts, in having a greater number of legs, the head soldered to the trunk, four antennæ, (in the great majority) and the composite eyes usually raised on moveable footstalks; and, in reference to internal structure, in possessing a complete circulation, and branchiae for respiration analogous to those of fishes. The only connection which the Annelides have with the preceding classes, arises from their annulated structure, the want of an internal skeleton, a similar nervous system, and in being oviparous; in all other respects they are widely removed from them. Their blood is red, like that of the vertebrata; the head is scarcely distinct, and there are no antennæ properly so called; none of them possess proper feet, and the majority are hermaphrodite.
Besides these distinctions, special to each individual class of the articulata, they all have this common difference from insecta, that they are destitute of wings, and do not undergo metamorphosis.[1] Their growth is gradual and insensible, during which many of them change their skins, but they preserve, with few exceptions, the same form they had at birth. Insects, on the contrary, pass through a variety of changes, during which they assume such dissimilar forms, that it is often impossible to recognise the same individual at different periods of its existence. The
- ↑ This assertion, however, must not in one instance be made absolute, for in regard to the Crustacea, a certain kind of metamorphosis may be assumed as having been recently demonstrated.
