The _Velella_ is a creature closely allied to the _Physalia_, having
the same essential structure, but differing greatly in form and
appearance. It consists of a flat disk of thin cartilaginous shell, of
a long rectangular outline, on which stands erect another similar plate
somewhat triangular, extending across from corner to corner. One large
polypite or mouth hangs down from the centre of the under side, and
short tentacles project on all sides from the edges of the horizontal
plate. The internal structure is very complex. The whole is enclosed in
a thin layer of flesh, which is tinged of a fine blue colour, sometimes
varied with purple and green. It is generally about two inches long,
and the erect plate rather less in height. As the little creature
floats on the waves, this plate forms a sail, and the breeze playing
upon it, imparts to the whole animal, according to Mr. Bennett, a
rotatory motion.[126]