their proper Order, much less in their proper Family. In the second order of Abdominalia (Pl. 23 and 24) the seventh or last cephalic segment is quite distinct, and bears rudimentary organs, answering to the first pair of maxillipeds of ordinary Crustaceans, of which organs, and of the segment supporting them, there is no trace in the Thoracica: the seven succeeding thoracic segments are destitute of any appendages; but the three segments of the abdomen bear three pairs of cirri. The mouth is peculiar in the labrum being developed into very large, moveable, lancet-formed organ; and the lower end of the œsophagus is armed with beautiful discs of teeth, and brushes of hairs,--a structure confined to this order. The male resembles the male of Alcippe; and the latter genus seems to be the connecting link between the Thoracica and Abdominalia. But the most important character of this latter order, in which it differs from Alcippe, and all other known Cirripedes, is in its metamorphoses; all the first changes are merely indicated by changes in form in an egg-like larva, without the development of distinct organs; and the last, or pupal condition, which is attained within the sack of the parent, is very peculiar, by the entire absence of natatory limbs. The third order of Apoda is the most peculiar of all; it contains, like the last, only one known species: the most acute naturalist, I am convinced, if he had not made the class his special study, would never even have suspected that this animal was a Cirripede. We see much magnified in Pl. 25, fig. 7 a naked, plainly-articulated animal, resembling the larva or maggot of a fly, attached by two threads; and these threads, on analysis, can be clearly shown to be the last rudiment of the carapace, specially modified. The last cephalic, the seven thoracic, and the three abdominal segments, are all equally destitute of appendages. The mouth is suctorial, and constructed on a plan unlike, I believe, anything known in the articulate kingdom; for the mandibles and maxillæ have rotated on their axes, and stand back to back; they can act only by tearing open a slit, and this action is performed in a hood, formed by the confluence of the broad palpi and labrum. Although the œsophagus is distinct, there is no stomach or anus. Lastly, owing to there being no carapace, the ova are developed, differently from in all other Cirripedes, within the thorax. I will close this preliminary discussion on the confines and type of the sub-class, by recalling attention, now that a sketch has been given of the three Orders, to the remark before made, that a wide range of structure is included within it, and by reurging that the Cirripedia should be ranked, not as one of the subordinate groups, but as one of the main divisions of the Crustacea. _On the Sexual Relation of Cirripedes._