Y.
EMBRYOLOGY AND LARVAL STAGE.
T he development of Nudibranchs, especially the Cladohepatica, has been studied by
various authors1 in the earlier stages up to the point where the animal leaves the egg
and swims about freely as a veliger covered with a shell. But the changes by which this form
becomes metamorphozed into an .55 olid or Dorid have hardly ever been observed. In fact
since the time of Schultze, who in 1849 saw the animal which he calls Tergipes lacinulatus
(probably = Galvina exigua A. & H.) pass from the veliger stage and assume the first
outline of its adult shape, no naturalist seems to have been able to keep veligers alive for
more than a short time,At any rate not in conditions sufficiently favourable to permit
further development. Neither I myself nor Mr. Nelson nor Mr. Elmhirst, who have
kindly endeavoured to rear the larvas of Doris tuberculata on my behalf, have ever been
successful in breeding the creeping form in captivity. The veligers often die soon after
hatching, and, though they may in some cases live as long as three weeks, they show no
sign of assuming the shape of the adult.
According to Smallwood2 the eggs of Nudibranchs are fertilized in the oviduct just
before deposition, the head of the sperm entering the egg, and the tail being left outside.
The period between copulation and egg-laying varies greatly in different species. In
some it is from twelve to twenty-four hours. In Doris tuberculata in captivity it is eight
days or more. The following notes refer to that animal. The eggs appear to remain
unsegmented for about twenty hours after they are laid, at the end of which time some of
them were observed to show the first cleavage.8 About sixteen hours later began the
second cleavage. Nine hours after that sixteen cells were visible in some eggs, and
again about eighteen hours later thirty-two cells. On the tenth day it could be seen
that the embryos had begun to develop* oral lobes, and a few days later that they
had assumed the form of veligers and were rotating within the capsule. Sometimes
there are two or three embryos in one capsule, in which case one usually develops a t the
expense of the others. The eggs hatch from fifteen to twenty-one days after they are
laid. I t is probable that the former is the natural period and that the movement of the
tide facilitates the process of hatching. It was materially accelerated in captivity when
the ribbon was gently agitated at intervals or placed under a stream of running water.
It is also possible that the operculum on the foot of the larva helps it to break through
1 Mona by Casteel; Tethys by Yigaier; Tergipes (?) by Schultze; several species by Trinchese
and Mazzarelli. See Bibliography.
3 Observations on Chromosome Vesicles in the Maturation of Nudibranchs, Sonderabdruck
aus : Morpholog. Jahrb., Bd. xxxiii, H. 1, Leipzig, 1905. The species studied are Gratena pilata, Cr.
gouldii, and Doris bifida. The last is unknown to me.
8 In this and all other questions of time it must be remembered that the unfavourable conditions
of captivity may retard processes which take place more quickly in the sea.
the egg-capsule. The larvae are able to eat almost as soon as hatched. The food
consists of unicellular algae, and the stomach and intestine soon become green and opaque
owing to the presence of chlorophyll. The internal organs are clearly visible through
the shell, particularly the attachment muscle. The veligers are positively heliotropic,
and if kept in a vessel which is partially screened so as to admit the light to only a small
portion, which is varied from time to time, they will persistently frequent the light portion.
They die, however, if exposed to the sun too freely, and also suffer greatly from the
attacks of various Infusoria. For one reason or another all died before casting off their
shells.1
The most detailed account of the development of a Nudibranch egg into a veliger is
that given by Casteel for Fiona. The first cleavage forms two cells of equal size, and the
same is recorded of Tethys and Ercolania, but in many Opisthobranchs (Acera, Aplysia,
Umbrella, and Philme) unequal cleavage is reported. In its earlier stages the egg is
radially symmetrical, but with the beginning of gastrulation this radial arrangement gives
place to an increasingly distinct bilateral symmetry. A gastrula is formed with a pointed
anterior end, and proceeds to develop into a veliger. The blastopore closes, but later the
stomodgeum opens at the point where it closed. The shell-gland appears first as an
invagination and then covers the posterior end of the veliger with a cap of large cells
which secrete the shell. The shell is found to be shifted slightly to the left even during
the earliest stages, and this asymmetry becomes more marked with the progress of growth;
torsion of the enteron also results from its being lengthened on the left side in consequence
of increased growth there. The foot arises as a swelling under the stomodseum, and later
secretes an operculum on its lower surface. There is no apical sense-organ, but cerebral
ganglia are present and also otocysts closely connected with the pedal ganglia; the eyes are
late in appearing. Casteel states that the larval kidney moves up to the right and eventually
lies above the anal opening; primitive excretory cells are also found in the body-
cavity. Kef erstem and Ehlers3 have given a short account of the embryonic development
of Bizzolia peregnna, which, t.o judge from their figure, commences with an unequal
cleavage of the egg.
It would appear from the various drawings which have been published that there is
no material difference of construction between the larvse of the different classes of Nudibranchs
or between any of them and the larvaa of Tectibranchs. Mazzarelli states; that'
those of Polycera and Chromodoris have eyes, whereas those of Elysia, Hermsea, Spurilla,
Antiopella, Fldbellina, Fiona, and Doto are blind. There would thus seem to be a distinction
in this respect between the Holo- and Cladohepatica. All have as their organ
of motion a broad, bilobed, ciliated velum, and all have a nautiloid shell, which lies
ventrally, the small foot and the lobes of the velum projecting out of it. It is generally
smooth, but in some, at any rate, of the Ascoglossa is simply sculptured.
One or more dorsal retractor muscles attaching the animal to the shell are usually
• i The general failure to rear Nudibranchs from the egg in aquariums must be due to far-reaching
causes. Probably the egg-ribbons laid by captive Dorids are not in perfect condition, and the poor
health of the embryos further deteriorates in consequence of the absence of tides and the abnormal
lateness of hatching.
3 Zoologische Beiträge, Leipzig, 1861.