DISTRIBUTION IN ZONES OF DEPTH.
1. Littobal Zone. 2. Cobbaline Zone. 3. Deep-watee oe Cobal Zone.
1 2 3 ■ 2 3
Doris tuberculata . . . . X Eolis glauca . . . . . X 1 0: ■;
flammea . . . ' . X X Alderi . . . . . X |
Zetlandica . . . . X? coronata -isSfl X v
millegrana . . . . X? Drummondi . . . . X X
planata . . . . X punctata . . . . X
Johnstoni . . . . X elegans. . . • H •
coccinea . . . . X rufibranchialis X X
repanda . . . . X lineata . . . . . X X
aspera . ! .. X smaragdina . . . . X
proxima . . . . X gracilis . . - . X
muricata . . . . X pellucida . . . . X
Ulidiana . . . . X Landsburgii . . . . X X
diaphana . . . . X alba . . . . . X x :
oblonga • X carnea . . . . . X
bilamellata . . . . X Glaucoides . . . . X
depressa . . . . X Peachii. . ! . • X
inconspicua . . . . X nana . . . . . X
pusilla . . . . . X stipata . . . . .
sparsa . . . . . •' X ; angulata . . . . X
pilosa . . . . . X 1 inornata . . .
subquadrata . . . . X concinna X
Goniodoris nodosa . . . . X olivacea . . . . X X
castanea X anrantiaca . . . X X
Triopa claviger . . . . X X pustulata . . . . , X .
iEgirus punctilucens X Couchii . . . . X
Thecacera pennigera . . X amcena . . . . Kj XX'
virescens X Northumbrica X
capitata . . . . X arenicola £*\Xl’.
Polycera quadrilineata X 1 Glottensis . . . X
ocellata . . . . X viridis . . . . X 5&5>x ■-
Lessonii . . . . X purpurascens X
Ancula cristata . . . . X"11 cingulata X
Idalia elegans . . . . X vittata . . . . X
Leachii . . . . X X ccerulea . . : X
aspersa. . . . . X exigua . . . . ; x ;
insequalis . . . . - X picta . . . . X X '
pulchella •X tricolor X
- quadricornis . X amethystina . X
Tritonia Hombergii X X Farrani X X
alba . . . . X despecta X
plebeia X Embletonia pulchra X
lineata X minuta X
Scyllsea pelagica X pallida X
Lomanotus marmoratua . X Fiona nobilis X
flavidus. X Hermcea bifida X X
Dendronotus arborescens X X dendritica X
Doto fragilis . . . . X X Alderia modesta X
pinnatifida X Proctonotus mucroniferus X
coronata . . ’ . X X X Antiopa cristata X X
Eolis papillosa X ? hyalina X
Alliances. At first sight the Nudibranchiate Mollusca appear to be a peculiar and
well-marked group, possessing characters that readily distinguish them from all the other
orders of Gasteropoda* A closer examination, however, shows that this is not the case.
The Nudibranchiata, though displaying in their typical forms a striking and well recognised
character, pass so gradually and almost imperceptibly into the neighbouring groups, as to
give rise to considerable difference of opinion on the limits of the order, and the genera which
should be included in it. In the present work, the order Nudibranchiata is restricted to those
forms with well-marked external branchiae which constitute the original Nudibranches of Cuvier.
The anatomical researches of later times have, however, shown so great a similarity in the
internal structure between these, and a small group of Mollusks, until lately nearly overlooked,
and the extent of which is still imperfectly known—our order Pellibranchiata, and the
Enterobranches remibranches and Bermobranches of Quatrefages—that an opinion is gaining
ground favorable to their constituting an aberrant group of the order Nudibranchiata. The
point of union between these two groups is found in the genus Elysia {Action of Oken), many
of whose characters approach very closely to those of Hermcea. In their external form the
principal difference consists in the want of distinct branchiae in Elysia, their place being
supplied by the extension of the sides of the back into longitudinal flaps or folds, that, physiologically
considered, may be supposed to represent the branchial papillae fused into a single
organ on each side. The head and tentacles in Elysia are similar to those of Hermcea, and
internally we perceive the same character of tongue and digestive organs, including a highly
ramified gastro-hepatic apparatus, which, in this genus, is probably carried to its greatest
degree of divisibility. The reproductive organs are also similar in both genera, if it may be
assumed that they are formed in Hermcea on the type of those in Calliopcea (Stiliyer), of which
there can be little doubt. Elysia has been considered by some authors to be a pulmoniferous
Mollusk, but this appears to be a mistake. With the exception of Placobranchus, a genus very
imperfectly known, the other Pellibranchs approach less nearly to the true Nudibranchs in
their external characters, though they retain considerable resemblance in their internal
organisation. Phyllirrhoe, a genus united to the Nudibranchiata on account of its anatomical
characters, by M. Souleyet and Dr. Gray, is a remarkably anomalous and aberrant form,
possessing scarcely any external appearance of relationship with this order.
On the other hand, the Nudibranchs show several points of affinity with the Tectibran-
chiata and Inferobranchiata of Cuvier, especially with the latter, two genera of which approach
very closely to the Nudibranchiate type. These are Phyllidia and Biphyllidia, both included
in this order by M. D’Orbigny and Dr. Gray. In the former genus we find a pustulated or
corrugated cloak covering the animal, and pierced, as in Boris, for the protrusion of the dorsal
tentacles and the anus. Like the Borididce, too, this genus exhibits a character very peculiar
among the Mollusca, that of having the skin stiffened with calcareous spicula. The branchiae
of Phyllidia, arranged in plaits around the lower margin of the cloak, in the groove between
it and the foot, show a relationship, in the character of these organs, to Patella and Chiton.
The genus possesses neither jaws nor denticulated tongue. Biphyllidia has the tentacles
* “ Destitute of any covering, and presenting a marked peculiarity in their external branchiae,
their appearance is so very different, that every unprejudiced observer cannot but wonder they should,
up to this day, have been mixed up with the true gasteropod shell-fish.”— Swaiuston, ‘ Malacology/
page 36.
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