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conspicuous, and several internal organs are visible, though they have been variously
interpreted by different observers. It is clear, however, that the mouth opens between the
foot and the head-lobes and leads to the stomach, from which issues an intestine terminating
in an anal papilla on the right near the lip of the shell. The liver consists of two portions,
differing in size (the left being the larger) and sometimes in colour. There are no special
respiratory organs, and most authors agree that there is no trace of a pericardium or heart
in the veliger stage, but opinions vary as to the kidneys. At any rate the kidneys are
not as in the larvaa of the Streptoneura, where they are thought to be represented by
caducous ectodermic projections. In the Opisthobranchiata the organs which have been
regarded as renal are closed pouches. This larva, known as a veliger from the two
conspicuous ciliated lobes of the velum, is a free-swimming organism, and does not begin
to creep until its structure has been materially modified.
I t is generally admitted that the whole class Euthyneura is characterized not merely by
the absence of the torsion found in the Streptoneura, but by detorsion, that is to say, that
an originally asymmetrical process of development is arrested and turned into a secondarily
acquired symmetry. In some archaic forms such as Actaeon and Ghilina the asymmetry of
the adult is still marked, but the Nudibranchs represent the opposite extreme, and such a
form as Dbris shows not only almost complete symmetry in the nervous system,1 but also
almost complete internal and external symmetry in the arrangement of the various organs,
only the genitalia (for which it would be hard to find a symmetrical position) remaining on
one side. The later development of the larvas is influenced by the fact that Nudibranchs,
unlike their relatives the Tectibranchs, do not grow over the shell until it becomes internal,
but break or cast it off at an early stage. Two methods of development may be seen in
those Opisthobranchs which, when adult, are really or apparently devoid of shells. In the
first the position of the organs is conditioned by the presence of the shell which in the
earliest stages of adult growth may be of relatively large size and hollow, although it subsequently
may become reduced to a flat plate or membrane. Hence, though detorsion
carries the ctenidium and anus some way down the right side, they cannot become postero-
dorsal, for they would then lie under the vault of the shell, which is not a suitable position
for them. Accordingly, these organs, as well as the renal opening and the osphradium, all
lie asymmetrically on the right, and the position of the heart and kidney is also more or less
asymmetrical. The mantle shows a tendency to grow over the shell as the animal becomes
larger, and may completely enclose it (e. g. Oscanius), but the distribution of the organs
mentioned is not altered. More rarely (e. g. in Plewrobranchaea) the shell is entirely absent
in the adult and apparently rejected during the larval stage, but growth proceeds as if it
were present, and a large asymmetrical ctenidium is developed on the right side. This
probably tends to keep the other organs in their places,1 2 for they are distributed exactly
as in Oscanius. But in Nudibranchs the shell is shaken off before any respiratory organs
are developed, and there is no limit to the animal’s changes of shape except its own plasticity.
In these circumstances it seems natural that symmetry should assert itself, though
it might be difficult to give any logical reason for holding symmetry to be natural. But
1 The symmetry is not so absolute as is generally supposed.
2 The anus is near the gills in most varieties of the molluscan organization. The gills, which
create a current of water, cause the excreta to be carried off and are not fouled themselves.
in any case there can be no doubt of the strong tendency in this group of Mollusca to
produce internal and external symmetry.
In the Nudibranchs there is never a ctenidium nor any organ corresponding to it
morphologically. Functionally, various new forms of gill take its place, and they are
always symmetrically arranged, either in the median line of the posterior dorsal surface,
or along the sides, or under the mantle. At the same time there is a tendency to make
the alimentary tract follow the longitudinal axis of the body, so that the mouth being
anterior the anus will be posterior. But this arrangement is never perfectly attained.
In most Cladohepatica the anus is lateral, in some Ascoglossa it is even anterior, and
even when it is postero-dorsal or terminal, as in the Holohepatica, the internal flexure
of the intestine testifies to an older arrangement. But the heart, liver, and kidney assume
symmetrical positions, and though the genitalia cannot do this as a whole, the hermaphrodite
gland in many forms spreads itself over the liver as a roughly symmetrical layer
or appears as one or more symmetrical masses. In both cases it ceases to be a dextral
organ. The conversion of the hermaphrodite gland into a layer illustrates another
feature of Nudibranch anatomy, which is possibly connected with the freedom of expansion
offered by the soft molluscan body when not confined by a shell, namely, the tendency to
convert into ramified systems organs which in other groups are compact masses or
simple pouches. This happens to both the liver and the kidney, and in the Ascoglossa to
some of the accessory genitalia. The ramifications, though incredibly complicated, are
roughly symmetrical, that is to say they extend themselves evenly as far as they find room.
The phases in which these tendencies find expression and convert the practically
uniform veligers into Nudibranchs of very diverse structure are still imperfectly known,
and the brief paper of Schultze (1849) “Ueber die Entwiokelung des Tergipes lacinulatus”
supplemented by Fischer1 appears not to be superseded. Nordmann’s “ Monographie
des Tergipes edwardsii ” (1845) also contains some account of the animal’s post-larval
development. The first change according to Schultze is the gradual obliteration of the
bilobed velum which begins on the second day and is completed in from nine to ten
hours. Then the shell and operculum are thrown off, there being some irregularity in the
different individuals as to whether these or the velum are the first to be completely lost.
At this time (about two days after leaving the egg) the jaws begin to form, but there is
no trace of the later ramification of the liver. The next important change is that the
visceral mass, which has previously formed a rounded hump separate from the foot,
settles down and forms an upper layer symmetrically disposed above the foot. The
animal has now become a creeping, not a swimming organism, and has the form of a
minute slug without tentacles or dorsal appendages. Jaws, radula, and eyes are well
1 Fischer in Ms 'Recherches sur la Morphologie du Foie des Gastéropodes/ Lille, 1892, treats of
the development of the same animal (under the name of Æolis eæigua) with special reference to the
liver. This organ consists of a right and left portion, of which the latter becomes much larger and
supplies the posterior as well as the left portion of the gastrohepatic system as it appears in the
adult.- He states that the growth and division of the liver in Æolids show that it is strictly
homologous to the liver of other molluscs, in spite of its different appearance. The contents of the
cerata are not to be regarded as glandular prolongations of the intestine but as lobules of the
original double liver which have become separated. His account of the order in which the cerata
appear is not quite the same as Schultze’s.