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In many Crustacea the carapace, formed by the backward production of the three anterior rings of the head, covers the dorsal surface of the thorax, and in some it encloses the limbs and mouth. This is likewise the case with the Cirripedia, and it is only the wonderful elongation of the anterior part of the head, its fixed condition, and the absence of external eyes and antennae, which gives to the Cirripedia their peculiar character, and has hitherto prevented the homologies of these parts from having been recognized. Cirripedia having a peduncle, flexible, and provided with muscles. Scuta furnished only with an adductor muscle: other valves, when present, not united into an immovable ring. Exactly analogous facts are presented, though more conspicuously, by the two species of the genus Ibla. Before examining this genus, the author had noticed the complemental males on Scalpellum vulgare, but had not imagined even that they were Cirripedia.