situated in advance of the frontal margin of the cloak; the anal opening is lateral, and the
skin is destitute of spicula. But this genus makes a near approach to the Nudibranchs in the
arrangement of the digestive apparatus, having a gastro-hepatic system, though the modification
of the parts is somewhat peculiar. In taking an extended view of the order, these two
genera ought perhaps to be included in it.*
There is yet another order with which the Nudibranchs might be supposed to come into
contact, through what, at first sight, appears to be an inosculating genus—the Pulmonata.
Among the naked slugs, the marine genus Onciidium (Peronia, Blainv.) shows some approach
to the Nudibranchiate tribes; but this is more apparent than real. In form it certainly very
much resembles Doris, and, though the specialised breathing organ is a pulmonary cavity
placed posteriorly beneath the cloak, yet there cannot be a doubt that, as in the Nudibranchs,
the skin partakes largely in this function. This we infer, both from its structure and from
the observed fact that the Onchidium Celticum has the power of remaining for a long period
under water, without the necessity of coming to the surface to breathe. In some of the
foreign species of this genus, the tubercles of the skin are prolonged, in the posterior part of
the cloak, into branched processes, very similar to the gills of Tritonia and Dendronotus.
This is remarkably the case in Onchidiumpunctatum {Peronia Alderi, Gray). There can be no
doubt, however, that Onchidium is organised on the type of the Pulmonata, whether we look
to the nervous, the digestive, or the reproductive system, as well as to that of respiration.
In their mode of development the Nudibranchs resemble very closely Aplysia, Bulla, and
most of those genera of Tectibranchiata and Inferobranchiata in which the process has been
observed. The spawn is likewise very similar. This is generally in the form of a gelatinous
riband, assuming more or less of a spiral direction. The larvae of Aplysia and Bulla are
scarcely to be distinguished from those of Tritonia and Doris. These tribes differ from the
Pectenibranchiata and the other orders in which the young come forth in a form resembling
the mature animal, principally in the circumstance that the embryo issues from the egg in an
earlier stage of development, and that the metamorphosis which it undergoes in all cases, takes
place within the egg in the one instance, and after its extrusion from it in the other. The
latter circumstance appears to imply a lower stage in the zoological scale, as it is found to be
the rule in the lower classes of animals, though a very rare exception in the higher ones.
Besides the more obvious affinities here mentioned, M. de Quatrefages has endeavoured
to show that a marked relationship exists between these Mollusks and most of the other classes
of Invertebrate Animals; that, in fact, the “Phlebenterata,” having, so to speak, thrown off
many of the characters proper to the class to which they belong, have at the same time
assumed others, typical of other great divisions of the animal kingdom. Thus, he traces in
these Mullusks characters that ally them to the Medusida, the Annelida, the Crustacea, and
even to the Infusoria. Believing as we do, that several of the characters given by M. de
Quatrefages to his t(Phlebenterataare founded upon erroneous observations, we cannot of
course agree in the conclusions drawn from them. If, for instance, as we believe, the idea
that the branched appendages of the stomach perform the three functions of digestion,
* Those authors who include the genera above mentioned.amongst the Nudibranchiata do away
with the order Infer obranchiata, and unite Pleurobranchus and the remaining genera of that order with
the Tectibranchiata into an order, distinguished by having the gills on one side only, which they name
Pleurobranchiata.
circulation, and respiration, be erroneous, the affinities of this tube with the Medusidæ must
fall to the ground. In the Planaria, too, the ramifications of the digestive system are
apparently very different from those of the Bolidida. M. de Quatrefages lays much stress
upon the symmetrical and bilateral character possessed by the Bolides, which, he conceives,
approach them very closely to the Annelida. We do not see much force in this observation.
Most of the organs of the Mullusca possess normally a bilateral structure ; and it is principally
in those parts of the Gasteropods which are usually inclosed in a spiral shell that the
arrangement is departed from. In proportion as the shell becomes a concave or flattened
disc, or entirely disappears, the animal generally assumes a more symmetrical character. In
few families, indeed, is this more observable than in some of the Bolidida, and in this respect
undoubtedly they may be considered to make a slight approach to the Annelida, as well as to
the other tribes whose bilateral structure is more perfect ; but this relationship cannot be
looked upon as of much importance; nor can we perceive any real resemblance to the
segment of annulose animals in the regular branching of the gastro-hepatic vessels. This
system in the Bolidida, to a certain extent, resembles the prolongations of the digestive
organ in the Nymphons ; but here also we see no reason for supposing that there is any true
affinity between the two groups.
The intimate relationship of the Nudibranchiata with the other Gasteropods having been
pointed out, it remains to inquire what rank they ought to hold in this Molluscan group.
The great concentration of the ganglions in the nervous system would appear to indicate a
higher place in the scale than is borne out by their other characters. The olfactory organs
also reach their highest development in some of the genera of this order. But, on the other
hand, the eyes, though formed on the usual type, being constantly buried under the skin,
must be less effective instruments of sight than in those orders where they are raised on
pedicels, and assisted by the modification and transparency of the epidermal layer. The
simplification of the special breathing organs in some of the genera, taken in connexion with
their entire disappearance in the nearly-allied Pellibranchs, and the great simplicity of form in
the latter, undoubtedly imply a lower degree of organisation than in most of the Gasteropods,
though by no means to the extent that has been assumed in those views, we have so often had
occasion to combat in these pages. The hermaphrodism common to the whole of the order,
and the peculiarities in their embryological development already pointed out, lead to the same
conclusion. Upon the whole, therefore, it will be necessary to assign to the Nudibranchiate
Mollusca a place amongst the lowest members of the class Gasteropoda.
Arrangement.—In reviewing the different arrangements of the Nudibranchiata, it will
not be necessary to go further back than the ‘ Regne Animal’ of Cuvier, with whom the order
originated. In that work, the “Nudibranches ” constitute the first order of the class Gasteropoda
in a descending series. The genera, which are not divided into families, stand thus :—
Doris, Polycera, Tritonia, Thethys, Scyllaa, Glaucus, Bolidia, and Tergipes. Very little alteration
was made by Lamarck in his ‘Animaux sans Vertèbres.’ He included these animals in a
single family,S'Zes Tritoniens,”—reversing the order of the genera to suit his ascending
arrangement* but still placing them first, consequently lowest, among the Gasteropoda,
though in Cuvier’s system they formed the highest group. Ferussac, adopting the orders as
well as the descending arrangement of Cuvier, divided the Nudibranchs into two sub-orders
and three families, as follows