XIV.

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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:—

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.'


                                                                                                                                                                                                                                                                                                           

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