Doris, Eolis, &c.
'The inhabitants of the watery element were made for wise men to
contemplate and fools to pass by without consideration.'--ISAAK
WALTON.
XIV.
One fine morning during the month of January, on peering into my
largest tank I perceived, attached to the upper portion of the marble
arch (centre piece), a peculiar object that had evidently been
deposited during the preceding night, but by whom or by what means I
knew not. It resembled a fungoid growth, or riband of flesh, plaited up
and attached at one edge to the stone. At every undulation of the water
the object moved to and fro with an extremely graceful motion.
By careful and close examination it was seen to be covered with a film,
that gradually expanded until it burst, and thus gave means of escape
to thousands of minute white granules. On submitting these to the
microscope, a most wondrous sight met my astonished eyes. Each dot or
granule proved to be a transparent shell, resembling the periwinkle or
rather the nautilus in shape; containing an animal whose excited and
rapid movements were amusing to witness. From out the opening of the
shell appeared now and again two rings of cilia. When these organs
were about to be put into action, they reminded me of two circular
tubes of gas connected together, and each containing innumerable
perforations, which were sometimes suddenly and entirely lit up by a
torch being applied to one end.
The _cilia_ may be distinctly seen to play at a certain point, and then
gradually extend round the circumference of the rings. When the whole
are in full action their movements are so extremely swift _as to appear
devoid of motion_, and thus bear a resemblance to rings of flame.
The result of the movements of the cilia was always evident in
the vigorous evolutions of the little embryos, for the microscope
filaments, while in action, caused the animal to roll about in all
directions in a confined circle. When this envelope burst, the little
nautiline dashed out, and then--then it was of little use attempting to
get a view of the animal again, so rapid and violent were its movements
to and fro, never resting for one instant on any spot, and least of
all the spot wished. By the aid of blotting-paper, I sometimes reduced
the quantity of water in the watch glass; and the animal, thus being
compelled to confine its evolutions to a narrower stage, was more often
within the field of view.
On visiting the sea-shore shortly after the discovery of the egg
cluster just described, I perceived attached to numberless stones and
large boulders thick clusters, composed of the self-same objects!
Beside them were lying confused heaps of _sea-slugs_, evidently
exhausted with their hatching exertions. Anything more repulsive to
the eye than those animal heaps exhibited it would be difficult to
conceive. Yet, at the same time, I know of no sight more pleasing than
to watch the _Doris_ in its healthy state, gliding along with outspread
plume on the under surface of the water, or up the sides of the tank,
more especially if it be observed through a powerful hand lens.
These remarks will perhaps convey some new information to the young
naturalist, embracing as they do the leading facts connected with the
wondrous embryotic development of many marine animals. The reader will
already be prepared to learn that the vivacious little animal, moving
by aid of cilia and enclosed in a _shell_, was in reality the youthful
stage of that slow creeping gasteropod the _Doris_, which, in its
mature form, is possessed of no cilia, nor any shelly covering whatever.
I should not have introduced this subject so familiarly did I not feel
anxious to make my readers aware how easy it is for each of them to
conduct experiments in the early stages of embryotic development, and
to gain practical evidence of the wonders which this study unfolds.
'What,' eloquently asks Mr. Lewes, 'can be more interesting than to
watch the beginnings of life, to trace the gradual evolution of an
animal from a mass of cells, each stage in the evolution presenting
not only its own characteristics, but those marks of affinity with
other animals which make the whole world kin? To watch the formation
of the blood-vessels, to see the heart first begin its tremulous
pulsations, to note how life is from the first one incessant struggle
and progress--these keep us with fascinated pertinacity at our studies.'
The remarkable fact above mentioned, of the young of the Nudibranchiate
Gasteropoda being furnished with a shell is exhibited not only in
Doris, but in Eolis, Tritonia, Aplysia, &c., while the embryos of the
Purpura, Nerita, Trochus, &c., are likewise in their youthful state
furnished with cilia, by the agency of which these animals swim freely
about in their native element.
There is one exception to this, which occurs in _Chiton_, the early
stage of which has recently been shown by the observations of Mr.
Clarke and Professor Loven to be peculiar, and more resembling that
of an annelid than of a mollusc. In this case the animal can scarcely
be said to undergo a metamorphosis; for the embryo, even within the
egg, has nearly the form of the parent, and the appearance of the
shell-plates is a mere matter of development.
I have never attempted to count the ova that were contained in any
single riband of Doris spawn, in fact I considered the task an
impossibility; but at a rough calculation, I concluded there would not
be less than a million. Dr. Darwin, however, travelling in the Falkland
Isles, met with a riband of spawn from a white Doris (the animal itself
was three and a half inches long), which measured twenty inches in
length, and half an inch in breadth! and by counting how many balls
were contained in a tenth of an inch in the row, and how many rows in
an equal length of riband, this gentleman reckoned that upon a moderate
computation there could not be less than six millions of eggs. Yet, in
spite of such amazing fecundity, this Doris was not common. 'Although,'
says Dr. Darwin, 'I was searching under the stones, I saw only seven
individuals. No fallacy is more common with naturalists, than that the
numbers of an individual species depend on its powers of propagation.'
This apparent paradox is not difficult of explanation when we consider
the number of enemies which are always hovering near, and ready with
hungry mouths to snap up the infant embryos as soon as they begin
to show signs of vitality. The Hermit-Crabs are especially fond of
Doris spawn, so much so, indeed, that the writer could never retain
any for hatching purposes while any of the Paguri were near. Mr.
Peach says they (the young Dorides) have myriads of enemies in the
small _Infusoria_, which may be noticed, with a powerful microscope,
hovering round them, and ready to devour them the instant weakness or
injury prevents their keeping in motion the cilia, which serve both
for locomotion and defence. Let them cease to move, a regular attack
is made, and the animal is soon devoured; and it is interesting to
observe several of the scavengers sporting with the empty shell, as if
in derision of the havoc they have made.
The same difficulty of calculation does not exist, at least to any such
extent, with the spawn of Eolis, which is laid in stringy coils. M.
Gosse mentions a specimen of _E. papillosa_ that laid nine strings of
spawn in his tank between the 20th of March and the 24th of May, all as
nearly as possible of the same length. Each string contained about a
hundred convolutions, each convolution about two hundred ova, and each
ova including, on an average, two embryos, making a total progeny of
forty thousand, produced from one parent in little more than two months.
I may mention that on no occasion have I ever found the spawn coils of
either Doridiæ or Eolididæ in my tanks, or at the sea-shore, except
during the months of January and February or March; neither have any
of my specimens spawned more than once during an entire season. From
noticing the same group of parent slugs congregated, and remaining,
as I can affirm, for weeks near their egg clusters, evidently in a
most enfeebled condition, it has occurred to me that on the Frith of
Forth, at least, vast numbers of these animals do not long survive the
hatching season.
Whether this be the case or not, it is a most singular fact that
in this locality, a Doris more than one or two inches in length is
scarcely ever to be met with.
There is at present in one of my tanks a specimen of the Doris of a
pearly-white colour, a second, tinted white and pink, and two others
which are quite _black_,--all being procured from the coast near
Edinburgh. The last-mentioned animals are, I think, somewhat uncommon.
When watching one of them in motion while the sun is shining down upon
it, the hue of the creature changes from a black to a very deep purple,
owing, no doubt, to its fleshy disc being many shades lighter than its
body, which, being extended, and exhibited under a full glow of light,
becomes semi-transparent. This peculiarity is not evident, of course,
when the Doris is lying in a passive state, with all its gill-plumes
closed up.
This sombre-coated gasteropod, although rare in some localities, is
very plentiful in foreign parts, if the following may be received as
an accurate narrative. 'On a reef of rocks near the island of Raiatea
is a huge unshapely black or brown slug, here called '_Biche_,' from
six to seven inches long, and five to six broad. Is is caught in vast
quantities, and not only regarded as a great delicacy by the natives,
but being cured, has become a valuable article of commerce in the China
market, whither it is carried from many insular coasts of the Pacific
by American ships. We have seen a number of lads fill three canoes in
two hours with these sea-snails.'
Thus uninviting as this slimy animal seems to our English taste, there
is evidently no doubt of its being used by the Chinese as an article of
food, and according to the evidence of certain authors, is esteemed by
the 'barbarians' a high-class luxury; but then we must remember that
the inhabitants of the land of gongs and chopsticks, have always been
famed for their singular gastronomic tastes. One poet writes:--
'That man had sure a palate covered o'er
With brass or steel, that, on the rocky shore,
First broke the oozy oyster's pearly coat,
And risked the living morsel down his throat.'
But, 'Mandarins and Pigtails,' what was such _risk_, I ask, compared
to that which _he_ endured, who swallowed the first mouthful of
birds'-nest soup? or horror of horrors, the first spoonful of sea-snail
stew? Yet we are told that both the 'mucilage' and the _Bêches de Mer_
dishes are savoury and highly grateful to the palate of an appreciating
gourmand.
A recent author, describing a Chinese dinner from personal observation,
tells us, that when the first dish, composed of birds'-nest soup,
was over, he waited the advent of the next course with very nervous
excitement. 'It was a stew of sea-slugs. They are slippery, and very
difficult to be handled by inexperienced chopsticks; but they are most
pleasant and succulent food, not at all unlike in flavour to the green
fat of the turtle. If a man cannot eat anything of a kind whereof he
has not seen his father and grandfather eat before him, we must leave
him to his oysters, and his periwinkles, and his craw-fish, and not
expect him to swallow the much more comely sea-slug. But surely a
Briton, who has eaten himself into a poisonous plethora upon mussels,
has no right to hold up his hands and eyes at a Chinaman enjoying his
honest, well-cooked stew of _Bêches de Mer_.
'During the discussion of this dish our Chinese master of the
ceremonies solemnly interposed. We were neglecting the rudiments of
politeness, no one had offered to intrude one of these sleek and
savoury delicacies, deeply rolled in sauce, into the mouth of his
neighbour. Efforts were made to retrieve the barbarian honour, but
with no great success, for the slugs were _evasive_, and the proffered
mouthful was not always welcome.'