In the male Ibla, abortion has been carried to an extraordinary and, I should think, almost unparalleled extent. Of the twenty-one segments believed to be normally present in every Crustacean, or of the seventeen known to be present in Cirripedes, the three anterior segments are here well developed, forming the peduncle: the mouth consists as usual of three small segments: the succeeding eight segments are represented by the rudimentary and functionless thorax, supporting only two pair of distorted, rudimentary and functionless cirri: the seven segments of the abdomen have disappeared, with the exception of the excessively minute caudal appendages; so that, of the twenty-one normal segments, fifteen are more or less aborted. The state of the cirri is curious, and may be compared to that of the anthers in a semi-double flower; for they are not simply rudimentary in size and function, but they are monstrous, and generally do not even correspond on opposite sides of the same individual. As males in other classes of the animal kingdom often retain some female characters, so here (though the case is not strictly analogous[50]) the male possesses the cementing apparatus, which homologically is part of an ovarian tube modified. [50] Certain plants offer a closer, though not perfect, analogy. Thus, in the florets of some compositous flowers, the pistil, besides its proper female functional end, serves to brush the pollen off the anthers; while, in the florets of some other compositæ (see the account of Silphium in ‘Ch. K. Sprengel Das entdeckte Geheimniss der Natur’), the pistil is functionless for its proper end, the flower being exclusively male, but its style is developed, and still serves as a brush. So in the male Ibla, part of the ovaria, in a modified condition, is still present, and serves as a cementing apparatus. The individuals in every other genus (with the exception of Scalpellum), in the several families, in the three Orders of Cirripedia, are hermaphrodite or bisexual. Why, then, is Ibla unisexual; yet, becoming, in the most paradoxical manner, from its earliest youth, essentially bisexual? Would food have been deficient, and was the seizure of infusoria by another and differently constructed individual, necessary for the support of the male and female organs? The orifice of the sack of the female is unusually narrow; would the presence of testes and vesiculæ seminales have rendered her thorax and prosoma inconveniently thick? Seeing the analogous facts in the six, differently-constructed species of the allied genus Scalpellum, I infer there must be some profounder and more mysterious final cause. 2. IBLA QUADRIVALVIS. Pl. IV, fig. 9. ANATIFA QUADRIVALVIS. _Cuvier._ Mém. pour servir ... Mollusq. 1817, Art. Anatifa, Plate, figs. 15, 16. IBLA CUVIERIANA. _J. E. Gray._ Annals of Philosophy, vol. x, New Series, Aug. 1825. ---- _J. E. Gray._ Spicilegia. Zoolog. Tab. iii, fig. 10.