these supra-oesophageal centres, is that which forms the third or great oesophageal collar (i) ■
It is stout, being composed of three parallel cords, and closely invests the oesophagus.
The infra-cesophageal ganglia are two pairs, symmetrically disposed on the buccal mass:
the large or buccal ones (e, e) are, as above related, connected, by means of a collar, with the
cerebroid; they are elliptical, and united across the median line by a stout commissure.
They give off laterally, and in union with the collar, two pairs of nerves, the sixteenth and
seventeenth, which are distributed to the buccal mass. The eighteenth is a small pair which
passes off backwards from the same ganglia to the base of the tongue, and is joined by the
buccal filament of the sympathetic. To the front of the buccal are attached the gastro-
oesophageal ganglia (ƒ,ƒ), which are very small, and give off three pairs of nerves: The
smallest of these, the nineteenth, is given to the salivary glands; the twentieth supplies the
top of the oesophagus, and communicates with the oesophageal sympathetic plexus. Lastly,
the two nerves constituting the twenty-first pair, by far the largest of the three, are continued
down the under surface of the oesophagus, nearly parallel with each other, communicating by
slender filaments with a fine open network of nerves and ganglia upon that tube, and unite
with two of the largest ganglia of the sympathetic system of the stomach.
The great supra-oesophageal ganglia vary a little in some species from those of
D. tuberculata, from which the above description is mainly taken. In D. pilosa and D. repanda
they are as distinct from each other as in that species, being only slightly altered in relative
position. In D. Johnstoni, D. cocdnea, D. bilamellata, and D. aspera, the cerebroid and
branchial are completely fused into one mass, which in some of these species is elongated in
the antero-posterior direction; in others, obliquely. No material variation takes place in the
origin and distribution of the nerves. The visceral ganglion is present in all.
The sympathetic or organic nervous system is extensively developed in D. tuberculata:
it is more or less demonstrable in the skin, the buccal mass, and in all the internal organs. It
consists of a vast number of minute but distinct ganglia, varying in size and form, the larger
quite visible to the naked eye, of a bright orange colour, like the ganglia round the oesophagus,
and interconnected by numerous delicate white nervous filaments, arranged in more or less
open plexuses or networks. This beautiful system is, at several points, as already indicated,
connected with both sets of cephalic ganglia, and is most completely displayed on the digestive
organs, forming at the cardiac extremity of the stomach a distinct circular chain or belt of
ganglia and commissures. Ganglia and nerves, forming an extensive plexus, are also well
developed on the reproductive organs, and the branchio-cardiac (PI. 1, fig. 2 q) portion of the
sympathetic is equally conspicuous.
Extensive traces of the sympathetic system have been detected in several other species,
as well as in D. tuberculata; but on account of their diminutive size, the ganglia and
connecting nervous plexuses could not be followed with the same degree of accuracy as in
that species.
The eyes of Doris are generally connected to the optic ganglia each by a very short
nerve, and are situated below the skin. In D. bilamellata, D. aspera, and D. pilosa, the nerve
is, however, somewhat elongated. The eye (PI. 2, fig. 14) evinces considerable development,
being furnished with a well-rounded black pigment-cup, having within the orifice a spherical
crystalline lens, which has in front an arched cornea, and the whole is enveloped by a transparent
membranous capsule. The auditory capsules (fig. 15) are always supplied with
numerous otolithes, and rest upon the cerebroid ganglia immediately behind the attachment
of the optic ganglia.
The skin in most species is tough and coriaceous, and is of a spongy or cellular structure
internally: the cloak in all the British species is stiffened with numerous imbedded, calcareous
spicula, having a more or less symmetrical arrangement. Spicula are also observable in the
foot, in the tentacles, and in the branchiae. The under surface of the foot, and the upper
surface of the cloak, as before stated, are covered with vibratile cilia. The branchial plumes
and dorsal tentacles are also furnished with them.
The ova of Doris (PI. 3, fig. 8) are generally arranged in transverse rows, imbedded in a
transparent gelatinous belt, attached by one of its edges to some foreign substance, and
coiled one or more times into a pretty regular cup- or vase-like form. In one division of our
second section, however, the ova are contained in a fine, depressed, close, thread-like coil.
The ova are very numerous. On a moderate computation, there cannot be less than 50,000
in a single patch of spawn (PI. 3, fig. 7) of D. tuberculata; and when we take into account
that all the individuals are prolific—the sexes being combined—and that each deposits two
or three patches in a season, it is evident how vast must be the number of young produced
each year, a single pair bringing forth probably 300,000 eggs.
Most of the species deposit their spawn in spring, and shortly afterwards the yolk, which
is contained within a delicate, transparent, membranous shell or chorion, may be observed
changing form (PI. 3, figs. 9, 10, 11, 12). The time required to mature the embryo varies in
different species, and probably in the same species, under different circumstances. The late
Dr. John Reid* found that the spawn of D. bilamellata was hatched in fourteen days, and a patch
of the spawn of D. tuberculata, removed by us from the rocks very soon after deposition, came
to maturity in fifteen days. The whole of the ova in the same mass of spawn, however, are
not hatched simultaneously, their successive liberation sometimes extending over a period of
several days. As the character of the embryo and mode of development are extremely similar
throughout the Nudibranchiate Mollusca, to avoid repetition, we purpose extending this
account of the embryology to the whole order, and may therefore here state that we have
ascertained that the spawn of Polycera quadrilineata is hatched in ten or twelve days; that
of Doto coronata in eight or ten, of Hermcea dendritica in nine, and of Eolis punctata in ten or
eleven. In all these cases the spawn was kept in the house, and probably the consequent
increased temperature may have hurried the development. Professor Nordmannf found the
development of his Tergipes Edwardsii to require from sixteen to twenty days.
In Doris tuberculata the chorion is elliptical, and generally contains one yolk, sometimes
two or thre e: at first the yolkj is round, and nearly fills the chorion (PI. 3, fig. 9); it soon
becomes a little elongated, with one end diagonally truncated (fig. 10); the truncated
end then becomes bilobed, each lobe exhibiting an imperfect spiral, and having its
margin ciliated (figs. 11, 12). The now animated being is seen to rotate within its prison.
Shortly, the lobes, which are placed in front, enlarge, and a fleshy process—the rudimentary
* Ann. Nat. Hist., v. 17, p. 388.
t Ann. des. Scien. Nat., 3d ser., v. 5, p. 144.
J The progressive cleaving or breaking up of the yolk has been minutely described by Dr. Reid.
Loc. cit.