external, is too anthropomorphic. It nnder-estimates the importance of one commonplace
factor.in the situation, namely, that the two objects which resemble one another (such
as the Nudibranch and the sponge or seaweed) have been associated and exposed to the
same influences for unnumbered ages. Thus, whatever may be the causes which tend to
give a sponge a certain appearance, they must also affect the Dorid which lives on the
sponge, at least so far as they are external and concerned with water, temperature,
and general surroundings.1 'The Dorid eats the sponge, and though- its colour is not
affected by its food so directly as in -Æolids, yet there is a connection between the two,
and Dorids deprived of food löse their colour. Further, though a Dorid is Very different
from a sponge in general structure, it does resemble it in some details ; the integuments
are spiculous, the back is often marked with little pits, and there are three openings for
the two rhinophores and the gills, which'resemble the oscula of the sponge. Thus the
Dorid, which is comparatively stationary for the greater partpf its life, lives among the
same conditions as the sponge, eats the sponge, and shows m the part of it which is
visible much the same structure as the sponge. That the two should become alike is not
surprising, and it is probable that Dorids becoming conspicuously unlike the supporting
sponge would be picked off and eaten by passing fishes. More difficult to explain
are the cases where a Nudibranch is coloured, not like the seaweed dr sponge which
| t habitually frequents and eats, but like its apparently chance surroundings, such
as a piece rock covered with nullipores and tubes of Spirortis. But varieties of
Dons tub&i'culata have been recorded to simulate perfectly the colour'of such an environ-
ment.
Other interesting cases of resemblance are afforded by the convergence in appearance
which-sometimes takes place between Nudibranchs and other groups^ Certain species,
of Psolus (Holothurioidea) are so like Dorids, and certain Turbellarians (especially
Polyelads with tentacles) are so like the Elysiidæ, that careful examination is necessary to
distinguish them, although in general organization the animals are very different, _Less
complete but still striking iS the resemblance borne by many Æolids, when slightly contracted,
to sea^anemones.1 * 3 Within the limits of the Mollusca may-be noticed the general
superficial resemblance between Lamellariidæ, Pleurobranchidæ, Oncidiidæ, and Dorids.
I have often mistaken the living Oncidlum savigm/i for a Dorid, for though it has a
respiratory chamber as usual in the genus, the processes on the back become elongate and
somewhat contractile just at the point where a Dorid would have gills. Some Polycerids,
such as Triopo clavigera, appear to mimic Æolids, such as Oalvina fanani.‘
In some of these cases the mimicking form may gain some advantage by resemblance
1 I t may be noted that not only do Dorids resemble sponges (where the resemblance is advantageous
to the Dorids) but also sponges resemble Dorids (where no such advantage to the sponge is
clear). I have often noticed this in small living tropical sponges, and an excellent illustration of it
will be found in the sponge figured in Delage and Hérouard’s ‘Zoologie Concrète,' vol. ii, Pt. i, PI. 12,
figs. 1 and 2. . . , .
s E.g. Æolidia papillosa to Sagartia troglodytes; Æolidiella alden to Sagartta sp.; Berghia
coerulescens to Aiplasia lacerata.
3 See Hecht 1, p. 577. As far as can he judged from preserved specimens, the Californian
Polycerid Laila cockerelli must he strikingly like an Æolis.
to a distasteful animal. iEolids appear to be disliked by fishes, and perhaps the brilliantly
coloured Polyelads have a similar immunity, whereas Eb/sia is known to be eaten by other
animals. But in the other cases it is not clear that either party gains any advantage from
the resemblance, and the shape may be due to mechanical reasons. A consideration of
Lamellaiia, Pleurtibranehis, Haris and Qneidium suggests that Gastropods of very different
types tend, when their shell becomes internal or is lost, to assume the same oval flattish
shape, especially if they are sedentary.. The general conditions of marine life probably
make it natural that a mass of soft flesh adhering by its base, not moving much and _ in
any case making no attempt to swim, should assume this form. More elongate and active
«nn.nl« such as Solids, Bolyceridse, Ascoglossa andAcclesia show a tendency to develop
processes and branches which is almost entirely absent in the squat oval forms. #milarly
there is a marked tendency illustrated in the most divergent groups (Coelenterates, Holo-
thurians, Cephalopoda, worms, etc.) for the body to assume an oblong form with a tuft of
processes at the end. This shape, unknown among land animals, clearly responds to the
conditions of marine lift,; Dorids habitually have their branchiae disposed in a tuft which
is terminal as far as their shape permits, and OwsiMum shows a tendency towards thjs
arrangement. Now Psolus is a specialized form which differs from other Holothunans m
having a clearly differentiated creeping surface. The possession of this sole makes it for
locomotive purposes a machine similar to Doris. The mouth and the circle of tentacles
surrounding it cannot collect food conveniently in a ventral position,1 and hence move up
to the dorsal region where they assume very much the position and appearance of the
•branchial rosette of Doris. Elysia and the Polyelads are both animals of- somewhat
unusual structure, namely a thin but fairly broad sheet of flesh which can both crawl and
■ |wim. It is not perhaps surprising'that similarity of appearance should,result from this
similarity of movement and external plan, although the internal differences of structure
are so great.
Some Nudibranchs which are distasteful to fishSs (and probably to other creatures) as
articles of food appear to be defended by a warning coloration. Prof. Herdman3 made a
series of experiments which pointed to the conclusion that amongjjjmmon British Nudi-
branchs the order of edibility is'Pendronotus, Doris, Aneula, and iEolids, the Solids being
the most distasteful form. Mr. Crossland also informs me that the fish of the Red Sea
reject blue Chromodorids. There can be no doubt that many brightly coloured animals
(such as Aneula and Okromodms) display themselves in a way which argues immunity,
but it is also true that they often live among equally brightly coloured Ascidians and
sponges, where, if not invisible', they at least attract no attention. But it is difficult to
attribute any special aim, either attractive or warning, to the beautiful colours of Nudi-
branohs, because a large number of the most brilliant-forms frequent localities like the
underside of stones, where they must be invisible to friend and foe alike. Their own
sight cannot do more than distinguish light and darkness, *so they cannot please one
another by their beauty. Also one form, flaabranchus, seems to be both protectively
I I t wonia seem that Psolm, like Cumrncjria, does not dig in the sand, but holds up a tuft of
tentacles, whioh it from time to time retracts, together with the small animals it may have collected
in them. . .
8 Third Report on the N udibranchiata of the L.M.B.C. District, 1890.