_Mouth._--The _labrum_ is always notched; sometimes it has no teeth, but generally there are three on each side; in _B. balanoides_ there are five or six on each side; and in _B. improvisus_ and _eburneus_ there is a whole row of teeth (Pl. 26, fig. 2, _e′_), graduated in size, on each side of the notch. The _palpi_ are large, with their apices nearly touching, and furnished with long spines. The _mandibles_ have, as it appears, normally, five teeth, but the two lower teeth are always small and often rudimentary, and almost confluent with the inferior, sometimes spinose angle. The _maxillæ_ have either a simple edge, or a notch under the pair of large upper spines, or the lower part forms (Pl. 26, fig. 7) a step-formed projection: there are generally two lower spines, placed singly or not in pairs, larger than the others, with the exception of the uppermost pair. The _outer maxillæ_ are, on their inner faces, obscurely divided into two lobes. _Cirri._--The rami of the first pair are unequal, the shorter one sometimes not being more than half the length of the other ramus: the segments of the shorter ramus are broad, and are, together with the lower segments of the longer ramus, thickly clothed with spines; in some species, as in _B. perforatus_, the anterior surfaces of the segments, more especially of the shorter ramus, and of both rami of the second pair are produced (Pl. 29, fig. 4), so as sometimes to form very remarkable projections. The segments of the second and third pairs are always thickly clothed with spines, as also are their pedicels. The third pair is rather longer than the second; but in _B. vestitus_ and _imperator_ it is much longer, and is otherwise somewhat different. The dorsal and basal margin of the pedicel of the third pair, in some of the species, as in _B. tintinnabulum_, is produced backwards on the thorax, and forms a membranous plate fringed with fine spines. The three posterior and longer pairs of cirri have from three to rarely eight or ten pairs of long spines on each segment, with generally one or two minute spines in the middle between each pair: their pedicels have a regular double row of spines. The _penis_ is long and hairy: in most of the species there is, at its dorsal basis, a small, sharp, flattened, imperforate projection; first observed by Poli: but this is sometimes absent, as in _B. crenatus_, though present in the closely allied _B. balanoides_; and its presence is variable in _B. tintinnabulum_. All the species have large plicated _branchiæ_. The base of the _sack_ in several species is furnished with inwardly projecting filamentary appendages. In _B. perforatus_, _crenatus_, and _improvisus_, and I believe in other species, the upper part of the _stomach_ is furnished with a circle of branching cæca.