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All the specimens met with by V. Siebold, and all the large ones which I have seen, were females. I observed, however, in the second specimen which came under my notice, that there was a small nematoid worm attached to the large female, Pl. 1, f. 1, A, near to the end in which lies the free extremity of the ovary. This minute worm was apparently overlooked both by Léon Dufour and V. Siebold; or, if they saw it at all, they probably mistook it for one of the ordinary young ones. It is always, however, in very close connexion with the female, the skins of the two being firmly attached to one another; and, if the small worm is torn away, there is a sort of rent at the spot where the attachment takes place. On the other hand, we know that in many nematoid worms the male is much smaller than the female, and the two are, during copulation, closely connected together; in Syngamus trachealis, indeed, this is so much the case, that the pair have been mistaken for a single animal. Moreover, although the small attached worm in Sphærularia is not altogether exactly like the ordinary young ones, still, in size and general appearance, it remarkably resembles them; and, lastly, unless we may regard it as being the male, that sex is, as yet, entirely unknown. Although, therefore, I have not been able to distinguish any generative organs, or trace of spermatozoa, I think that I am justified in considering that in Sphærularia the male is far smaller than the female, and that the two are fastened together in a certain definite manner and position. The shape of tail is also quite different from that of the larva; in Pl. 1, f. 6, I have represented one of the young worms; and in f. 7, one of the attached specimens; and it will be seen that the tail is quite dissimilar, being straighter, and more pointed in the latter. Of what nature, then, is this minute worm, and what are its relations to the large female Sphærularia? Three possibilities only occur to me, viz.: that it might be the larva, a parasite, or the male.

There is, however, no instance in the Nematoidea of any such mode of metamorphosis; and the little creature, though quite motionless, looks too fresh and transparent to be merely the shrivelled-up skin of the young. The difference of shape just alluded to, also, militates against this view, which is, I think, quite untenable.

Nor is the parasitism of the little creature a more probable supposition. In the first place, the almost, if not quite, invariable presence of the little worm speaks against it; and, secondly, the mode of its attachment is almost equally conclusive, as no Nematoid worms are external parasites. Moreover, it is evident that this little worm must perish at the same time as, or soon after, the Sphærularia, and it is equally clear that in the month of July this latter has not long to live; if, therefore, the