plates, seven in a row on each side, and can be distinguished from that of P. oceUata only by
the external plates, which are a little squarer and broader than in that species: the two
spiriiferous plates next the median line are perfectly alike in both these forms, and the
tongue is divided down the centre by a naked space, in precisely the same manner.
The nervous system has three pairs of oesophageal ganglia, arranged as in Polycera, only
the cerebroids are a little elongated in the antero-posterior direction, and the buccal ganglia
are closely united together.
The nerves appear to be distributed as in that genus ; the eye is provided with a lens
and cornea, and is sessile on the cerebroid far forward on the outer border; the auditory
capsules contain numerous otolithes, and seem to belong to the same ganglia.
The skin is fine and smooth;—the spicula have not been examined.
Genus 6. POLYCERA,* Cuvier.
Corpus limaciforme, aut lseve aut tuberculatum, postice acuminatum, m palliatum. Caput
subinferius,' subteutaculatum, velo frontali mucronato vel tuberculato, in lineam dorsalem tuberculatam
utrinque continuato; maxillis comeis. Tentacula 2 dorsalia, subclavata, laminata, non-retractilia.
B ro n c h ia plumosae, in lined media dorsi anum cireumdantes, appendirabus linearibus vel lobatis utrinque
ornatee. Apertura genitalis ad latus dextrum.
With great uniformity in general appearance, the Polycerina shew so many varieties in
detail that it ;is difficult to avoid raising almost every species to generic rank. The genus
Polycera was instituted by Cuvier, in the ‘ Règne Animal,’ on the type of the Doris quadrilineata
of Miiller, a species of which, though reappearing from time to time, under many different
names__cormta,flam, lineata, varions, and typica—still remains :the only animal in which the
Polycerd (many-homed) character of the frontal expansion is distinctly recognisable. Several
other species have been included in the genus, nearly all of which have since been detached to
constitute different genera, and the two which form our second section might without impropriety
be raised to a similar rank, were it not that we are unwilling to multiply genera on
slight characters. Thè moat available characters for generic distribution in this sub-family
are the tentacles—laminated or unlaminated—sheathed or unsheathed—with or without basal
filaments. The appendages adorning the branchial region are also óf importances as well as
the more or less distinct cloak, sometimes only indicated by a dorsal area circumscribed by a
ridge of tubereles or filaments, and occasionally almost entirely wanting. Cuvier thought
that the branchial lobes covered and protected the branchiae in times, of danger, but most-of
these appendages are but ill adapted for such a purpose.
Themisto of Oken is synonymous with Polycera. The range of the genus is rather uncertain,
but it appears to extend throughout the European seas, and probably over most of the
shores of the Atlantic. Five species have been found in Norway, two in France, and one in
the Mediterranean. Of extra European species, one occurs in the Canary Islands, another on
the coast of North America, and a third in Rio de Janeiro.
The body of Polycera is limaciform, smooth or tuberculated, swelling a little in the centre,
and acuminated behind. The head is subinferior and has the sides a little lobed or dilated
without forming true oral tentacles. The mouth is furnished with corneous jaws, a rare circumstance
in the Doridliee. The veil, as it is usually called, or, more properly, the frontal
t * From ttoXuç, many, and icspaç, a horn.