which is present in a rudimentary state in the Polycerince and a few of the Eolidida, is
nothing more than a fold of thé dorsal skin, and as such may be looked upon as representing
the so-called organ in the testaceous gasteropods; though perhaps its true homologue exists
in the side-lappets of the TrocMda; in which case these latter organs must be members of the
dorsal skin, and not of the foot.
H abits. As it is seldom possible to study these animals in their native haunts, the little
that we know of their habits is mostly gathered from the observation of individuals kept in
confinement, and consequently under influences more or less artificial.
Some of the species are nocturnal, but this is not the case with many of the littoral
tribes. Such of these as are gregareous on small sea-weeds, as Polycera quadnlineata, may
be observed in a state of activity during the day time, in tide pools left among the rocks, and
apparently enjoying the warm rays of the sun. Alderia modesta has a similar habit in more
shallow water, becoming almost amphibious; and Doris bilamellata is frequently found
exposed on rocks left dry by the tide. But the greater number of the species avoid the light,
concealing themselves under stones and shelving rocks. Most of the littoral tribes are found
near to low-water mark; though some few kinds occur much higher up among the rocks,
where they must remain several hours every tide deprived of water. Doris pilosa, Polis
papillosa, and E. nana, are generally met with in such situations; even the spawn of these
species is frequently left dry by the receding tide.
The Nudibranchs partake largely of the sluggish character of the class to which they
belong. This is. more especially the case with some of the Dondidce and Prztoniada, which
will remain for hours fixed to a spot without apparent motion, and when roused from their
torpor by the desire of food or some other stimulus, crawl slowly from place to place. Some
of the Polyceritus are an exception to this rule, and many of the Eolidida, more especially those
of our second section, are very active and lively in their movements. Dr. Johnston remarks respecting
the Gasteropods generally, that the narrower and more elongated the foot, the quicker
the motion, which becomes retarded just as that organ tends more to the oval or round. This
observation holds good with respect to the Nudibranchs, if we except two or three genera in which
the foot, from its extreme narrowness, is little fitted for progression. In these genera, as Glaucus,
Scyllaa, and Doto, the foot is formed for clasping the stems of algae or corallines, the sides
being very thin and flexible, anacd apble of being brought together so as to embrace the
stem: the foot can be completely flattened when applied to a plain surface. The Eolides with
a very narrow foot move slowly on a flat surface, probably for the same reason.
Crawling is the usual mode of progression with these animals. This is effected in the
manner of the snail, by a series of minute undulations of the under surface of the foot, arising
from the alternate relaxation and contraction of the pedial muscles. None of our native
species have the power of swimming freely through the water; and we are not aware of any
means they have of reaching the surface but by crawling up any substance in contact with it.
This they do frequently in confinement, by ascending the sides of the vessel, and then
launching themselves, with outspread foot, on the surface of the water in an inverted position.
Like most of the other aquatic gasteropods, they are very fond of floating in this way, which they
do without any apparent effort. The mode of progression of the mollusca in this position has
been the subject of some dispute, and it is undoubtedly somewhat obscure. In the opinion of
M. de Q-uatrefages and some other naturalists, it is produced by the action of the vibratile
cilia which cover the surface of the body. Dr. Johnston objects to this, that it does not explain
all the phenomena: for instance, “An Eolis crossing a basin can at once stop and remain
there for some time; but during all this period of rest, the cilia are in as active a state as
when the creature is in motion.”* This is undoubtedly the case, and would appear fatal to
M. de Quatrefages’ theory; and, moreover, the shell-bearing gasteropods are devoid of the
same extent of ciliated surface, and yet they float with equal celerity. It has also been
supposed that the undulating motion of the sides of the foot, acting against the water, is
sufficient to account for the progression of the animal when floating, but this explanation
appears equally unsatisfactory. We think, after carefully examining the subject, that this
mode of progression is not very dissimilar from that of crawling. Whilst floating along the
surface of the water, the sole of the foot is constantly undulating, as if moving upon the
ground; and as a considerable quantity of mucus is always floating from it, the motion is
probably produced by the undulations of the foot acting against this mucus, which forms a
track on the surface of the water behind the animal. The inverted animal walks, as it were,
along the floating mucus, much in the same way as it glides over the mucus which it sheds on
its path when crawling; but in the latter case, the mucus, becoming adherent to the ground,
enables the foot to act with greater effect than it can against the floating mucus: hence the
animal can always crawl more quickly than it can float.
While floating in this manner the Nudibranchs occasionally drop suddenly down,
suspending themselves from the surface by a thread of mucus, which is fixed to the tail or
posterior extremity of the foot. In this way they will let themselves gradually down to the
bottom, or remain some time pendant in the water without apparent support; for the thread
of mucus is so transparent that it can scarcely be seen. When carefully looked for, however,
it can always be perceived, originating in the track of mucus left on the surface by the
animal; the mucus forming a small inverted cone at the point from which the thread issues,
and here slightly dimpling the surface of the water. This thread of mucus must not be
confounded with the byssus of the bivalves. Some of the species also occasionally suspend
themselves from the surface of the water by the hind part of the foot, which for this
purpose is expanded into a disc, and from this, as a fixed point, the body is suspended and
moved about at the will of the animal. In this case the floating mucus is the fulcrum to
which the hind part of the foot is attached.
On any of these occasions, either when floating or when suspended, if alarmed, the
animal falls at once to the bottom. This is effected by the foot quitting its hold of the mucus,
when the animal, being specifically heavier than the water, of course sinks: its specific gravity is,
no doubt, increased by the collapse of the parts; but the Nudibranchs are never lighter than
the water, even when fully expanded, though in this state many of them are nearly buoyant.
We have seen Dendronotus arborescens float for a considerable time in the middle of the
water without any apparent connexion with the surface. This comes nearest to the act of
swimming that we have observed in any of the native species. Some foreign genera, however,
can swim freely through the water in any direction. Tethys and Melibe are stated by
M. Sander Rang to do this by means of the large veil with which they are furnished, assisted
by the undulating motion of the posterior part of the body; and the same means of progression
* * Introduction to Conchology/ p. 130.