developed. The stomach shows two or three caeca and the anus is median and posterior.
The animal begins to eat hydroids. On the fourth day appear the two rhinophores in
front and two cerata behind, containing diverticula of the alimentary canal. The anal
papilla lies between them.1 Twenty-four hours later appears a second pair of cerata, in
front of the first. At this stage no cnidosacs are present, but the alimentary diverticula
are formed at the same time as the cerata. The cerata continue to grow, usually
appearing by pairs, at first only in front of the first pair, but subsequently also behind.
When four to five pairs have appeared, accessory lateral cerata begin to be protruded
until the front groups consist of two or three cerata each. The heart is very late in
making its appearance, and is never formed until three or four pairs of cerata with their
liver diverticula have been developed. Schultze states that it sometimes does not
appear until four weeks after hatching. Nordmann, on the contrary, states that the heart
may be seen clearly beating in Tergipes edwardsii before the velum is lost, and that the
oral tentacles are visible at the same time; also that the first cerata to appear are the
most anterior pair, all subsequent pairs being posterior. But there is no reason to
suppose that the development of all JEolids need be uniform in its details.
The remarkable Pseudovermis described by Kowalevsky might be supposed to be an
early stage of an .ZEolid before the cerata or hepatic diverticula are developed, especially
as no heart was discovered with certainty. But, apart from the facts that it is said to lay
eggs, the presence of cnidosacs is against this hypothesis.
Of the phases through which Dorids pass after the veliger stage practically nothing
is known, efforts to rear D. tiiberculata from the egg having, as already mentioned, not
proved successful Garstang2 states that the young JEgires punctilucens is pure white
with a series of very spiny tubercles, and only later acquires brown pigment. He collected
a complete series of these young forms, which enabled him “ to. identify as a still earlier
stage of the same species a remarkable post-larval form which had occurred in the autumn
tow-nettings as observed by Mr. Bles at Plymouth and Mr. Vallentin at Falmouth. It was
simply one of these minute white JEgires with the addition of a pair of large velar lobes
with which it swam freely in the water.” I t would appear from this that the animal had
assumed the form of a Dorid when free-swimming and still retaining its velum. But it
may be observed that JEgires punctilucens is not remarkably doridiform even when adult.
The smallest Dorids which I have seen are about a millimetre in length, and colourless.
The tubercles and the spicules are both relatively much larger than in the adult D. tuber-
culata, though in making this comparison it must be admitted there is no absolute proof
that the young forms belong to this species. As far as can be seen in such minute
objects the branchiae and their pocket are distinctly developed. At a later stage when
the animal measures 4-5 mm. in length the dorsal tubercles are still large, and spots of
brown pigment appear between them.
The exceptional development of Genia coclcsi has been described by Pelseneer.3 I t is
the only Nudibranch which is known to spend its larval stage within the egg and to quit
, 1 In the adult Galvina exigua the anal papilla is latero-dorsal between the second and third rows
of cerata.
3 “ Faunistic Notes at Plymouth during 1893-4,” Journ. Mar. Biol. Assoc., vol. vii.
3 “ La condensation chez un nudibranche,” Trav. Stat. Zool. Wimereux, vii, 1899.
EMBEYOLOGY AND LABVAL STAGE. 35
the egg in the adult form. Compared with most other Nudibranchs this species lays very
few eggs (six to twelve) at a time, which.in captivity hatch after eighteen days. The velum
does not attain to more than a rudimentary stage of growth and is then obliterated
before hatching, but at no stage in the process is there any sign of a shell-gland or a shell.
The foot lengthens, uniting itself with the visceral mass, and the hepatic diverticula begin
to form. It is only when these changes are complete that the animal leaves the egg,
having attained the adult form except that the tentacles and genitalia are not yet
developed.