The male, immediately after its metamorphosis from its pupal condition, which has been fully described, is almost globular, but slightly bilobed, and is formed of strong, structureless, transparent membrane, including a mass of cellular matter, apparently without any included organs: it is attached by about the middle, between the anterior and posterior lobes, by the not-moulted prehensile antennæ. When the male is mature, its greatest length, measured from the posterior end, where the orifice is seated, to the anterior and blunter end, is about 13/1000 of an inch, and therefore rather less than the pupa, which was 16/1000ths in length. Relatively to a full grown female, the male slightly exceeds half the diameter of the toothed orifice leading into her sack, see (z) fig. [1], Pl. [23]. In the mature condition, (fig. [19]), one lobe, namely, the upper or posterior, has become more pointed, and is terminated by a minute orifice, 8/6000ths of an inch in diameter. This orifice is formed by a rim of thickened brownish membrane, which, on what was the ventral surface, has a few very minute, but strong, sometimes bifid spines;—in this one character, the male resembling the female. The other and lower (homologically anterior) end or lobe is broader, and contains a mass of cellular matter, which, from its close resemblance in appearance and position to similar matter within the male [Alcippe], I have no doubt forms the contents of the testis. In one single specimen, I succeeded in isolating a vesicula seminalis of small size, containing perfectly distinct spermatozoa. Across the middle, between the two lobes, close under the outer integument, there is a broad layer of rather strong transverse muscular fasciæ. I did not observe any eye, the presence of which I should have expected from analogy. Internally there is no mouth, thorax, cirri, or other organs, excepting the testis and vesicula seminalis just mentioned, and an immensely elongated probosciformed penis, coiled up and filling the rest of the inside of the sack down to the testis, which latter occupies the whole anterior, and generally lower end of the animal. This penis is plainly articulated, and includes fine transversely-striated muscles: no doubt it can be protruded through the minute orifice, and voluntarily moved about. Out of a male, 12/1000ths of an inch in length, I dissected a penis, which, when not stretched, measured 50/1000ths of an inch in length; when a portion was pulled between two needles, it could be stretched to apparently three times its former length, and I should think that this organ could be extended by the animal to, perhaps, even the 100/1000ths of an inch,—that is, to between eight and nine times its own entire length! The use of this enormously elongated penis obviously is, that the spermatozoa of these males, which are so extremely small in size, compared to the female, should all be conveyed within the sack, and none be lost. It should be borne in mind, that the whole male, including every part, is scarcely larger than a single ovum, of which sometimes sixty have to be impregnated by only two or three males. In a full-grown female, the distance from one of the attached males to the middle of the orifice leading into the sack, is about the 5/100ths of an inch, equal to the length of the coiled up, not-extended penis: the further distance from the orifice of the sack to an ovum lying at the bottom of the sack, would be almost 10/100ths of an inch, so that the spermatozoa have to pass a distance of 15/100ths of an inch from the testis of the male to the lower ova. I believe two thirds of this distance would be passed safely along the probosciformed penis.
The resemblance between the male of [Cryptophialus] and of [Alcippe] is truly surprising; and is the more wonderful, considering the great dissimilarity of their pupæ. Hardly any characters can be pointed out in which these males differ, excepting such as might have been thought of only specific value, namely the relative proportions of the different parts, and mere external shape. The peduncle growing a little after the metamorphosis, in the male of [Alcippe], and the prolongation of its capitulum with the included oblique ligamentous fibres, are the greatest differences. Having fully remarked, under [Alcippe], on the wonderfully rudimentary condition of these males, destitute as they are of so many parts and organs, I will here say nothing further on these singular creatures, destined to discharge their spermatozoa, die, and be succeeded by a fresh set of short-lived male successors.
Order III.—APODA.
Cirripedia, with the carapace reduced to two separate threads, serving for attachment: body consisting of one cephalic, seven thoracic, and three abdominal segments, all destitute of cirri. Mouth suctorial, with the mandibles and maxillæ placed back to back, enclosed in a hood, formed by the union of the labrum and palpi. Metamorphoses unknown.
The characters above given fully justify, I think, the formation of this order; though it contains only one species, the [Proteolepas bivincta]. The mere external appearance (Pl. [25], fig. [7]), so wonderfully different from that of every other cirripede, would by itself prompt to this same conclusion. At first sight the [Proteolepas], if of fresh-water origin, might even have been mistaken for the larva of some insect, fastened by two threads to its prey. The entire absence of the three anterior segments of the head and therefore of the carapace, or, speaking strictly, the mere rudiment of these parts, forming an envelope to the two cement-ducts,—the absence of a stomach, rectum, and anus,—the entire absence of thoracic and abdominal appendages or cirri,—the absence of a probosciformed penis,—are all negative characters, which might ensue from degradation, so common with parasites; and which might, therefore, have been esteemed of not high classificatory value. But the suctorial mouth, with the palpi and labrum united into a hood, and with the mandibles and maxillæ reversed or turned back to back, so as to be utterly incapable of prehension, is a type of structure not hitherto met with, I believe, in any other animal, and cannot be explained away by degradation. The formation of the ova within the segments of the body, a peculiarity confined to this one cirripede, evidently results from the non-development of the anterior part of the head, within which the ova are usually formed; but the compound structure of the vesicula seminalis is a peculiarity which cannot be thus explained. [Proteolepas] has no particular affinity to any other cirripede; it resembles, indeed, [Cryptophialus] in one important point, but only in one point, namely, in the number of the segments of its body. It is really beautiful to see how the homologies of the archetype cirripede, as deduced from the metamorphoses of other cirripedes, are plainly illustrated during the maturity of this degraded creature, and are demonstrated to be identical with those of the archetype Crustacean. I was at first inclined to rank [Proteolepas] in one division, and all other cirripedes in another division of equal value; but as it may be inferred from the characters of the prehensile antennæ, that the pupa did not differ much, if at all in any important character, from the pupæ of other cirripedes, I have thought the three orders, which I have instituted, would be the most natural arrangement. As any one looking at the drawing given of [Proteolepas], might very naturally feel inclined to protest against its being ranked as a cirripede, I must reurge the importance of the pupal antennæ being constituted on the common type, for from their structure, by the law of correlation, that of the whole pupa may be inferred; and even still more I must insist on the importance of the one great character of the antennæ being cemented to the surface of attachment by matter proceeding, as we shall see, in a modified state, from the great ovarian sack. The structure, also, of the mouth (to a certain extent), the segmentation of the body, though in appearance so peculiar, the hermaphrodite condition, the single penis, the absence of oviducts, all accord with, and taken together demonstrate, its cirripedial nature.