powers as the Nudibranchs should be ordained to make perilous migrations, while the well
protected testaceous gasteropods are permitted to enjoy a sedentary existence. The latter
might roam about in comparative security; and, indeed, trusting to their shelly covering, they
do boldly expose themselves on rocks and sea-weeds. The Nudibranchs, on the contrary,
avoid the light, their nakedness compelling them to seek concealment in crevices of rocks and
under stones, where they probably pass the whole of their lives; and being thus, as it were,
confined to a spot, their dissemination is provided for by the natatory condition of the larvae;
without which provision these animals would probably be extremely local. The way in which
the species are distributed between tide-marks is also unfavorable to the notion of migration.
as most of them inhabit well defined zones or belts on the beach, beyond which they are rarely
found.
The periodical appearance of the Nudibranchs within tidal range admits of another and
more satisfactory explanation; namely, that these animals, whose period of life is probably
short, may not live longer than a year, and that most of the old individuals die off after the
breeding season, and the young fry, being for many months afterwards small and inconspicuous,
escape observation, attaining maturity only a short time before the next breeding season,
when, having given birth to a new progeny, they die and disappear in their turn.
Whatever be the cause, the various species generally do disappear from the rocks shortly
after their respective breeding seasons, though for some months afterwards solitary individuals
occur. In the course of two or three months the fry may be detected if closely looked for,
and afterwards their development noted until they reach maturity and shed their spawn in the
same localities that were occupied by their parents. We have in this way traced the
development of Goniodoris nodosa throughout the year, and can vouch for its attaining its full
growth in twelve months, as, indeed, is evident enough in most of the common rock species.
It is true that the disappearance of the adult individuals after spawning may be accounted for
by their retiring into deeper water; but, in this case, we might expect to be able to trace their
change of place on the shore at different seasons; besides that, several of the most abundant
littoral species are scarcely, if ever, brought up by the dredge, and certainly never in such
numbers as to warrant the conclusion of their living for the greater part of the year below low-
water mark. Some of the species are to be found within tide-marks during the whole year,
though most plentifully in the breeding season. Such species most probably live longer than
one year. Other species, again, are taken within tidal range for a year or two, and then
disappear from the locality for a long period. Some of these species appear to belong to deep
water, and their accidental occurrence on the rocks may arise from the agency of currents
sweeping the natatory larvae beyond the limits of their proper zone.
Not much is known concerning the longevity of the Nudibranchs, but if the opinion we
entertain respecting the periodical appearance of these Mollusks on the rocks be correct, it is
evident that the period of existence of numerous species must be short,—not much exceeding
a year. And, judging from the rapidity of their growth, and their great fecundity, compared
with the few individuals that are found in a mature state, it may be inferred that such is correct.
In some of the species, however, individuals of overgrown size and venerable appearance may
be met with that might be supposed to have lived through many summers. M. Bouchard
Chantereaux states that Eolis Cuvien (papillosa) does not acquire its full growth till the end of
the second y ea r; and M. Nordmann thinks that his Tergipes Edwardsii lives at least two years.
The period will, no doubt, vary in different genera and species, though in none is it perhaps
much extended.
The Nudibranchs, notwithstanding that they are androgynous, frequently copulate during
the breeding season. The conjoined individuals lie side by side, their heads turned in opposite
directions. Thus the right sides of the two animals are brought into close contact, and mutual
impregnation is effected. They remain in this position for some time, and in a short period
after separating, generally about the first or second day, the spawn is deposited. This is
usually adherent to some foreign body, and on expulsion from the animal is enveloped in a
perfectly transparent mucus, which at first is tenacious, having the property of adhering to
whatever it comes in contact with; but the surrounding water soon destroys this quality, and,
to some extent, hardens it. By means of this mucus, the spawn becomes at once glued, as it
were, to the surface of whatever substance the animal may happen to be resting upon. While
shedding the spawn it slowly and gradually moves backwards in a spiral direction, beginning
in the centre; thus the spawn assumes the spiral form, coiling in the usual way from right to
left. Occasionally, it is found coiled in a contrary direction, in which case the animal must
crawl forward during the propulsion of the spawn* The spiral form is the most perfect when
the animal is resting upon a flat surfaceon corallines, for want of an expanded support, it
necessarily becomes more irregular* The form and caliber of the sexual aperture and the even
or interrupted propulsion of the spawn, combine in different degrees with internal organic
causes, to form those peculiar and elegant patterns which characterise the different species.
When the spawn is shed freely and at once, the masses assume an oval or kidney shape; but
a more slow and gradual propulsion results in its taking a spiral form from the causes already
named.
The time required for the maturity of the embryo after the deposition of the spawn varies
in different species, and under different circumstances, from a few days to a month or more;
the usual time appears to be about ten days or a fortnight. The extraordinary difference between
the minute natatory larva in its first state of existence and the adult animal, has been fully
described in another part of this work.f That a minute creature, scarcely visible to the naked
eye, enclosed in an operculated shell, and swimming freely through the water by means of
ciliated lobes, should turn into the large and sluggish Loris tuberculata or Tritonia Hombergii,
is one of nature’s romances, only to be learnt from a careful study of her works. Unfortunately,
though their development from the spawn can readily be traced, naturalists have not hitherto
succeeded in preserving these delicate little creatures alive for more than a few days, so that
the progress of their change into the perfectly formed animal has not been traced in detail.
Thousands, no doubt, perish at an early stage, or become the prey of other marine animals. The
accidents to which they are liable, or the occurrence of circumstances favorable to their preservation
and diffusion, may readily account for the occasional appearance and disappearance
of some species in a manner apparently, at first sight, somewhat arbitrary.
* The figures of the spawn in different works, in consequence of the inattention of the engravers,
are often reversed, and cannot be relied upon.
•j- See the genus Doris, anatomy and development.1