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The theory afterwards received a good deal of support from the research made by a number of distinguished zoologists and anatomists, especially C. Kupffer, B. Hatschek, F. Balfour, E. Van Beneden, and Julin.
The account was published in a most respectable Journal, that of the Royal Society of Belgium; but I could not avoid feeling doubts I hardly know why, except that there were no accidents of any kind, and my experience in breeding animals made me think this very improbable. So with much hesitation I wrote to Professor Van Beneden, asking him whether the author was a trustworthy man.
The existing deficiencies were the more difficult to supply, because, as Van Beneden remarks with regard to the Decapoda, from the often incredible difference in the development of the most nearly allied forms, these must be separately studied usually family by family, and frequently genus by genus nay, sometimes, as in the case of Peneus, even species by species; and because these investigations, in themselves troublesome and tedious, often depend for their success upon a lucky chance.
For a discussion of this subject, see P. van Beneden, Commensaux et Parasites, Paris, 1875. The Fierasfer, a little fish of the Mediterranean, installs himself in the respiratory cavity of a Holothurian; he does not live at the expense of his host's flesh, but contents himself with levying a tax on the foods which enter the cavity.
Indeed, it is only within the last thirty years that the splendid patience of Von Siebold, Van Beneden, Leuckart, Küchenmeister, and other helminthologists, has succeeded in tracing every such parasite, often through the strangest wanderings and metamorphoses, to an egg derived from a parent, actually or potentially like itself; and the tendency of inquiries elsewhere has all been in the same direction.
Sexual conjugation thus remains a process which is radically the same as the non-sexual mode of propagation which preceded it. The fusion of the nuclei of the two cells was regarded by Van Beneden, who in 1875 first accurately described it, as a process of conjugation comparable to that of the protozoa and the protophyta.
Do not Nauplius-like states in this case also precede the Zoea? The last segment and the last two pairs of feet of the middle-body are wanting. The developmental history of Mysis, the near relationship of which with the Shrimps and Prawns has recently again been generally recognised, has been described in detail by Van Beneden. So far as I have tested them I can only confirm his statements.
Beyond the vermin that infest the skin and the hair, tapeworm, and a few other intestinal worms, little if anything was known of morbific parasites before the Nineteenth Century; but the labors of Van Beneden, Küchenmeister, Cobbold, Manson, Laveran, and others have now established the causal relationship between great numbers of animal parasites gross and microscopic and certain definite morbid states.
But it is not my object to give all the classifications of different authors here, and I will therefore pass over many noted ones, as those of Burmeister, Milne, Edwards, Siebold and Stannius, Owen, Leuckart, Vogt, Van Beneden, and others, and proceed to give some account of one investigator who did as much for the progress of Zoölogy as Cuvier, though he is comparatively little known among us.
This appears to be only inconsiderable in the common Lobster, the young of which, according to Van Beneden, are distinguished from the adult animal, by having their feet furnished, like those of Mysis, with a swimming branch projecting freely outwards. From a figure given by Couch the appendages of the abdomen and tail also appear to be wanting.
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