'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.'