A third genus is that of the ostracoderms or 'testaceans'. These
are animals that have their hard substance outside and their
flesh-like substance within, and their hard substance can be shattered
but not crushed; and to this genus belong the snail and the oyster.

The fourth genus is that of insects; and this genus comprehends
numerous and dissimilar species. Insects are creatures that, as the
name implies, have nicks either on the belly or on the back, or on
both belly and back, and have no one part distinctly osseous and no
one part distinctly fleshy, but are throughout a something
intermediate between bone and flesh; that is to say, their body is
hard all through, inside and outside. Some insects are wingless,
such as the iulus and the centipede; some are winged, as the bee,
the cockchafer, and the wasp; and the same kind is in some cases
both winged and wingless, as the ant and the glow-worm.