In the sack with these two males, there were certainly four, I believe five, larvæ, which in every main point of structure resembled the larvæ of other pedunculated Cirripedes. From the peculiar form of their prehensile antennæ, differing in no respect, except in the proportional lengths of the segments, from the same organ in the male I. Cumingii, I can feel no doubt that these were the larvæ of the male I. quadrivalvis;—for a moment’s reflection will show how excessively improbable it is, that several larvæ of some other Cirripede, and that a Cirripede intimately allied to the parasitic male Ibla, should have forced themselves, without any apparent object, into the sack of the hermaphrodite Ibla. The larvæ, though not yet attached, were on the point of attachment, so that the single eye of the mature animal could be distinctly seen, lying near to the two great compound eyes of the larva. We have also just seen, that one male quite recently here had undergone its metamorphosis. The larvæ are 25/1000ths of an inch in length, and rather more than 10/1000ths in width in the widest part: they are boat-shaped, the dorsal edge forming the keel of the boat; the anterior end is only a little blunter than the posterior end; the quasi-bivalve carapace is smooth. All the essential points of structure in the larvæ of other Cirripedes at this stage, could be distinctly here seen,—such as the two compound eyes, with the apodemes to which they are attached, and the two oblong sternal plates whence the apodemes spring,—the adductor muscle,—the six natatory legs, with long plumose spines,—the abdomen, with its three small segments and the caudal appendages,—the prehensile antennæ already described,—and, lastly, the two little (auditory?) sacks at the antero-sternal edges of the carapace, but not so near the anterior extremity as in Lepas. The four or five larvæ, after having undergone in the open sea the several preparatory metamorphoses common to the class, must have voluntarily entered the sack of the hermaphrodite: ultimately would they, on finding two males already attached there, have retired, and sought another individual less well provided; or would they all have remained, and so formed a polyandrous establishment, such as we shall presently see occurs sometimes in Scalpellum? This must remain quite uncertain.

In this same hermaphrodite specimen of I. quadrivalvis, the two ovigerous lamellæ contained some hundreds of larvæ in the first stage of development, which were liberated from their enveloping membranes by a touch of a needle: they were about the 16/1000ths of an inch in length, and presented all the usual characters of larvæ at this period. What a truly wonderful assemblage of beings of the same species, but how marvellously unlike in appearance, did this individual hermaphrodite present! We have the numerous, almost globular larvæ, with lateral horns to their carapaces, with their three pair of legs, single eye, probosciformed mouth and long tail:—we have the somewhat larger larvæ in the last stage of development, much compressed, boat-formed, with their two great compound eyes, curious prehensile antennæ, closed rudimentary mouth and six natatory legs so different from those in the first stage:—we have the two attached males, with their bodies reduced almost to a mouth placed on the summit of a peduncle, with a minute, apparently single eye shining through the integuments, without any carapace or capitulum, and with the thorax as well as the legs or cirri rudimentary and functionless:—lastly, we have the hermaphrodite, with all its complicated organisation, its thorax supporting six pairs of multi-articulated two-armed cirri, and its well-developed capitulum furnished with horny valves, surrounding this wonderful assemblage of beings. Unquestionably, without a rigid examination, these four forms would have been ranked in different families, if not orders, of the articulated kingdom.

Concluding Remarks.—If the creature which I have considered as the male of [Ibla Cumingii] be really so, and the evidence formerly given seems to me amply conclusive, then the animal just described, from its close affinity in every point of structure with the former, assuredly is the male of [Ibla quadrivalvis]. But feeling strongly how improbable it is, that an additional or complemental male should be associated with an hermaphrodite, I will make a few remarks on the only possible hypothesis, if my view be rejected,—namely, that the two parasites considered by me to be exclusively males, are not so, but are independent hermaphrodite Cirripedes, the female organs and ova (which, if present, would have been nearly mature, judging from the presence of spermatozoa in both species) having been overlooked by me in every specimen: and again, that in the animal described as the female I. Cumingii, I have, though minutely dissecting several specimens, and finding far smaller parts, such as the organs of sense and nervous system, entirely overlooked all the conspicuous male organs, though when I came to I. quadrivalvis, and naturally expected to find it likewise exclusively female, a single glance showed me the great probosciformed penis, and by the simplest dissection the vesiculæ seminales and testes were exhibited. Such an oversight is scarcely credible; but even if assumed, we have to believe in the extraordinary circumstance of the two parasites being species of an independent genus, not only the very next in alliance to the animals to which they are attached, but in certain most important points, namely, the organs of the mouth, actually deserving a place in the very same genus. Moreover, the two parasites differ from each other, not only in about the same slight degree, but in a corresponding manner, as do the two Iblas to which they are attached; thus the mouths of [Ibla quadrivalvis] and I. Cumingii are closely similar, (the difference being barely of specific value,) so are the mouths of the two parasites; but the parts are larger in the hermaphrodite I. quadrivalvis, than in I. Cumingii, so are they in the parasites. Again, the most conspicuous character in I. quadrivalvis, is the number of segments in the caudal appendages, far exceeding those in the other species of Ibla, as well as of every other pedunculated Cirripede, and the parasite of this species has articulated spinose appendages, far larger than the barely visible, non-articulated pair in I. Cumingii.

Considering the whole case, there seems no room to doubt the justness of the conclusion arrived at, under the former as well as under the present species, namely, that these little parasites are the males of the two species of Ibla to which they are attached;—wonderful though the fact be, that in one case, the male should pair with an hermaphrodite already provided with efficient male organs. It is to bring this fact prominently forward, that I have called such males, Complemental Males; as they seem to form the complement to the male organs in the hermaphrodite. We look in vain for any, as yet known, analogous facts in the animal kingdom. In the genus Scalpellum, however, next in alliance to Ibla, in which, consequently, if anywhere, we might expect to find such facts, they occur; and until these are fully considered, I hope the conclusions here arrived at, will not be summarily rejected. Although the existence of Hermaphrodites and Males within the limits of the same species, is a new fact amongst animals, it is far from rare in the Vegetable Kingdom: the male flowers, moreover, are sometimes in a rudimentary condition compared to the hermaphrodite flowers, exactly in the same manner as are the male Iblas. If the final cause of the existence of these Complemental Males be asked, no certain answer can be given; the vesiculæ seminales in the hermaphrodite of [Ibla quadrivalvis], appeared to be of small diameter; but on the other hand, the ova to be impregnated are fewer than in most Cirripedes. No explanation, as we have seen, can be given of the much simpler case of the mere separation of the sexes in [Ibla Cumingii]: nor can any explanation, I believe, be given of the much more varied arrangement of the parts of fructification in plants of the Linnean class, Polygamia.

Genus—Scalpellum. [Pls. V], [VI.]

Scalpellum. Leach. Journ. de Physique, t. lxxxv, July, 1817.

Lepas. Linn. Systema Naturæ, 1767.

Pollicipes. Lamarck. Animaux sans Vertebres, 1818.

Polylepas. De Blainville. Dict. des Sc. Nat., 1824.

Smilium (pars generis). Leach. Zoolog. Journal, vol. 2, July, 1825.