12 VARIATION AND DISTRIBUTION.
For instance, the species of Doto and Favorimis are more brilliant and variable in the
Mediterranean than on our coast, Marionia is more gorgeous than Tritonia, the modest
Aldisa zetlandica and Geitodoi'isplanata are replaced by the red A. berghi and G. reticulata.
South of about 40° N. red and yellow Dorids are common (e. g. Blatydoris argo, Baptodoris
cinnabarina, Doriopsilla areolata, Plocamopherus maderse), but no forms have yet been
discovered showing such extraordinary brilliance of colour as Hexabranchus, Ghromodoris,
Miamira, etc., in the Indo-Pacific.
A glance at the map will show that Nudibranchs living near Greenland and Spitzbergen,
or their larvae, can be carried to Behring’s Sea by the north coast of either Europe or
America without being exposed to any unaccustomed temperature. The passage between
the two oceans is even easier round the extremities of Africa and South America. But the
migration of a marine animal from the tropical Atlantic to the tropical Pacific necessitates,
not only a long journey, but changes of temperature which would probably be fatal to all
but pelagic or deep-sea species. Hence it is not surprising if the Nudibranchs (and, indeed,
all the Mollusca) of the tropical Atlantic and of the Mediterranean have little in common
with those of the tropical Pacific. Some pelagic forms, such as Scyllsea, Glaucus and Fiona,
seem to be really cosmopolitan. Platydoris argo is thought by Bergh to occur in the
Indo-Pacific. Discodoris notha, Phyllidiopsis papilligera, Thecacera maculata and Nem*
brotha gratiosa are very like D. concinna, Ph. carinata, T. maculata and N. affinis from
tropical Pacific waters. But in no case is the identity certain. Goniodoris castdnea is
recorded from the Atlantic, the Mediterranean, Suez, and New Zealand, but its presence
at Suez is no doubt attributable to the Suez Canal.
The same want of complete equivalence seems to extend to the generic as well as to
the specific divisions. Of the Atlantic species referred to such genera as Discodoris,
Thordisa and Trippa, many are marked by their authors with a query or are admitted to
possess abnormal characters. Three of the Dorids from South Africa described by
Bergh are left without any precise generic designation because they do not fit in to any
recognized group. The fact is that the scheme of classification used for tropical Dorids
was devised for the Indo-Pacific forms and suits those of the Atlantic very imperfectly.
The west coast of America is, as a faunistic region, distinct from the Indo-Pacific.
Information as to the Nudibranchs found south of California is not copious, but Ghromo-
doris seems rather scarce, and the characteristic Indo-Pacific genera, such as Hexabranchus,
Phyllidia, Bornella, Melibe, are not mentioned. The genera recorded (if they
can be regarded as representative) indicate a general resemblance to the" Atlantic fauna,
though most of the species are different. The chief of them are Archidoris, Anisodoris,
Diaulula, Euplocamus, AEolidia, Phidiana and Gratena. For the coasts of California and
Alaska we have more details as to the distribution of Nudibranchs, which resembles that
prevailing on the eastern side of the Atlantic in several interesting points. Dendronotus,
Tritonia, and Solids are well represented, the latter both by special types and by
Atlantic genera, such as AEolidia and Flabellina. The Polyceridse (Polycera, Ancula,
Thecacera, AEgires, and several new genera) and the Pseudodorididae (Acanthodoris,
Lamellidoris, Adalaria and Akiodoris) are very numerous. Cryptobranchiate Dorids are
represented by Archidoris, Aldisa, Bostanga, Gadlina, Diaulula, Discodoris, and allied
genera. On the Californian coast appear a few species of Doridopsis and Ghromodoris,
the latter extending as far north as Puget’s sound (about Lat. 49° N.). Thus, exactly as
on the eastern coast of Europe, we have a northern fauna characterized by iEolids,
Dendronotus, Polyceridse, Pseudodorididse, and such cryptobranchiate Dorids as Archidoris
and Gadlina. A little further south some forms, such as Dendronotus, cease to be recorded,
and the southern genera Doridopsis, Ghromodoris and Discodoris begin to appear,
extending rather further north than their known Atlantic range.
The following north Pacific forms are considered specifically identical with Atlantic
Nudibranchs, though most of them are also given varietal names: Archidoris tuberculata,
Dendronotus arborescens, Acanthodoris pilosa and Lamellidoris bila/mellata, whereas the
species of Adalaria, Gadlina, AEgires and Polycera are regarded as separate, but nearly
allied to A. proxima, G. repanda, AE. punctilucens and P. lessonii. There is not much
difference between varieties of the same species and nearly allied but distinct species.
They both represent successive stages in the same process, namely, that a northern
circumpolar fauna becomes slightly differentiated in the two oceans.
Closely allied species of Diaulula and AEolidia, or perhaps varieties of the same
species, are recorded from the southern Atlantic and from California. Though the
fauna of New Zealand contains many forms unknown to the northern Atlantic and
northern Pacific, it includes animals described as Acanthodoris pilosa, Doris tuberculata,
Goniodoris castanea, and a species of Drepania not very different from those found in the
Atlantic and Mediterranean. But the identity of the species mentioned is not beyond
dispute.
Thus the distribution of those Nudibranchs which inhabit the littoral zone or moderate
depths seems,f,q depend mainly on climate. The faunas of the northern and southern
Atlantic as well as those of the northern Atlantic and northern Pacific show considerable
similarity in genera and even in species. Some species are common to the north
Atlantic and south Pacific, and are probably cosmopolitan in temperate waters.
lElolidia, OhlamyUa, Dendronotus, Gadlina,1 all the Pseudodoridid®, and several genera of
the Polycerid® are almost or quite unrecorded in the tropics, and, though occurring in
temperate regions, are most numerous in cold waters; exactly the opposite is true of
Ghromodoris, Doridopsis, Discodoris, Marionia,2 and others. Dorids are much more
abundant in tropical than in c’old waters, and AEolids seem to be less abundant. But
the fact that they are less conspicuous, and are not numerous in the accessible localities
(e. g. reefs) where Dorids abound, may have something to do with this apparent scarcity.
All collections made in or near Arctic and Antarctic waters show a considerable preponderance
of Gladohepatica, particularly ASolids. There is less resemblance between the tropical
faunas of different oceans than between the temperate faunas, because for geographical
reasons intercommunication between the temperate seas is easier. Scanty as are our
records for the tropical Atlantic, they indicate a great discrepancy in genera with that
large part of the tropical Pacific which comprises India, the Red Sea, East Africa, N.
Australia, Polynesia, and the Malay Archipelago. With the West Coast of tropical
1 Tyrmna also, a genus allied to Cadlina, is recorded from temperate waters: Chile, Lat. 41 S.
a The known species of Marionia are recorded only from the warmer seas, but Tritonia, which
replaces it in northern waters, is also found (though scantily) in the tropics.