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.