These, with the difference in colour, constitute the chief distinctive characters, but there is
also observable some little difference in their habitats. On the Northumberland coast, where
it is not very rare, we have never found this species but within tide-marks; P. Lessonii, on
the contrary, is generally found in deep water on the same coast, and is common upon
corallines brought in by the fishing boats; but never, among the many specimens which we
have obtained by this means, have we found a single individual of P. ocettata.
At Torquay we found this species in pools among the rocks near high-water mark.
A curious light-coloured variety was brought to us among the produce of a day’s
dredging in Dublin Bay, where it appeared to occur in tolerable abundance: we did not
learn at what depth, but from the other contents of the dredge we conclude that it was
in shallow water. This variety had the dark portion of the surface much reduced in size,
and forming little more than a network round the spots. Another variety occurs on the
Northumberland coast; it is of a pale yellowish green entirely devoid of spots, and might
easily be confounded with the young of P. Lessonii, but for the tentacles and branchial
plumes, which are always sufficient to distinguish the species.
The Triopa Nothus of Dr. Johnston is most likely the young of this species; but, from
the imperfect state of the specimen from which it was described, it would be impossible to
speak with certainty.
This animal floats frequently, and can fix itself by its tail with great firmness to any
substance over which it may be passing. It is sometimes difficult to detach it from the
polished surface of glass. Its heart beats from seventy-two to eighty-eight times in a
minute. Calcareous spicula are distributed through the skin, but are not crowded nor
placed in regular order. There are two kinds, one much larger than the other: the larger
is cylindrical, obtuse at the ends, a little bent in the centre and pretty regularly covered
with irregular circles of tubercular points; the other kind is scarcely one third the size,
smooth, cylindrical, slightly bent in the centre and thickened and rounded at the ends.
The spawn occurs in July and August—occasionally in September; and is generally of
a delicate rose colour and of a strap form, attached by its edge to stones : it is usually two
or three times coiled, sometimes not so much, with the coils rather wide apart and angulated.
The eggs are of an oval form, and mostly in transverse rows.
Fig. 1, 2, 3. Polycera ocellata, different views.
4. A branchial plume, very highly magnified.
5, 6. Side and front views of a tentacle, much magnified.
7. A portion of the skin, showing imbedded spicula.
8, 9. Spicula, more highly magnified.
10. Spawn.
11. A portion of the same more highly magnified.