Moreover

the animals that are unfurnished with shells grow
spontaneously, like the testaceans, as, for instance, the
sea-nettles and the sponges in rocky caves.

Of the sea-nettle, or sea-anemone, there are two species; and of
these one species lives in hollows and never loosens its hold upon the
rocks, and the other lives on smooth flat reefs, free and detached,
and shifts its position from time to time. (Limpets also detach
themselves, and shift from place to place.)

In the chambered cavities of sponges pinna-guards or parasites are
found. And over the chambers there is a kind of spider's web, by the
opening and closing of which they catch mute fishes; that is to say,
they open the web to let the fish get in, and close it again to entrap
them.

Of sponges there are three species; the first is of loose porous
texture, the second is close textured, the third, which is nicknamed
'the sponge of Achilles', is exceptionally fine and close-textured and
strong. This sponge is used as a lining to helmets and greaves, for
the purpose of deadening the sound of the blow; and this is a very
scarce species. Of the close textured sponges such as are particularly
hard and rough are nicknamed 'goats'.