[53] According to M. Joly, ('Annales des Sciences Naturelles,' 2d series, tom. xix, p. 59) in the larva of the macrourous Caridina, the natatory legs appear before the gnathites or parts of the mouth; so that in ordinary Crustaceans there is no invariable order of development from the anterior towards the posterior end of the body, as has sometimes been supposed. The mouth is more or less probosciformed (Pl. 29, figs. 8-10, _d_), differing considerably in this respect in different species of the Lepadidæ; and this, probably, is due to the larva being born in a more or less mature condition. Its exact position likewise varies, for it arises either between the first or second pairs of natatory legs. It is known, from Mr. Bate's observations, to have the power of movement. It is directed posteriorly, the œsophagus extending anteriorly; both these directions being the same as in the mature cirripede. Certainly during these early stages there are no jaws or gnathites; but the margin, answering to the labrum, is furnished with some short, thick, sharp spines, and with hairs. In _Scalpellum vulgare_ the orifice of the œsophagus seems to lie rather beneath the upper prominent spinose edge, which, as just remarked, probably answers to the labrum; but this is one of the species in which the probosciformed mouth, at least before the first moult, is not much developed. We come, now, to the three pairs of natatory legs: the first (Pl. 29, figs. 8-10, _e_) has throughout the order only one ramus, whereas the two succeeding pairs (_f_, _g_) are biramous. I must here remark that the straight and strong, and the curved plumose spines, with which these limbs, after the first moult, become furnished, now appear to me as more probably prehensile, rather than masticatory as I imagined in my former volume. That these spines are important organs to the larvæ I do not doubt. With regard to the homologies of these three pairs of limbs, my first impression was that they were the mandibles and the two pairs of maxillæ in their earliest condition; but I consider this view as quite untenable, for several reasons; viz., the wide interval between their bases and the mouth itself,--the somewhat variable position of the mouth with respect to the legs,--and the position which the latter occupy in the _second_ larval stage.[54] A far more tenable view is that these three pairs of legs are the three pairs of maxillipeds, in their earliest condition, in accordance with the view of M. Joly[55] on the nature of the three very similar pairs of natatory legs in the larva of Caridina, a macrourous Crustacean. But, in Cirripedes, the three pairs of natatory legs, in the larva in the first stage, are apparently the very same as the first three pairs, in the larva in the second stage, and in the pupa. And in the pupa the first three pairs, which certainly correspond with the first three pairs of cirri in the mature animal, seem to me, for reasons