As the lamellæ are formed without organic union with the parent, they would be liable to be washed out of the widely open sack of the Lepadidæ, if they had not been specially attached to the _fræna_. These fræna consist of a pair of more or less semicircular folds of skin, depending inside the sack, on each side of the point of attachment of the body. The fræna are often of considerable size, but in Ibla, they are very minute; they are formed of chitine tunic with underlying corium, like the rest of the sack; on their crests, there is a row, or a set of circular groups, or a broad surface, covered, either with minute, pointed, bead-like bodies mounted on long hair-like footstalks, or with staff-formed bodies on very short footstalks. I measured some of the bead-like bodies, in _Lepas anserifera_, and they were 1/2000th of an inch in diameter, and the footstalks three or four times as long as the elongated heads. These heads, of whatever shape they may be, have an opaque, and, I believe, glandular centre; I could not make out with certainty an aperture at their ends, but, I believe, such exists, and they seem to secrete a substance, which hardens into a strong membrane, serving to unite the crest of the frænum to the edges of the lamellæ. In one case, this bit of membrane seemed formed of a woven mass of threads. These little glandular bodies, with the membrane formed by them, are cast off at each exuviation, and new glands formed on the crest of the frænum underneath. In some species of Pollicipes, (viz., _P. cornucopia_ and _elegans_,) the fræna, though present and large, are functionless and destitute of the glands: I believe, they exist in this same functionless condition, and in rather a different position in the sessile Cirripedes, and that in this family they serve as Branchiæ.