These are situated in the same position as in the Lepadidæ, namely, in a slight swelling on the sides of the thorax (Pl. 25, fig. 1, _d′_) just beneath the basal articulation of the first pair of cirri. The orifice in Tubicinella and Xenobalanus is slightly produced, or is tubular; the free part in the former genus projecting 5/100ths of an inch. The structure of all the parts is essentially the same as in the Lepadidæ, but I think all are proportionally larger. The external membrane of the body is turned inwards at the orifice, as a short flattened tube, which widens considerably (being, in a middle-sized specimen of Coronula, 4/100ths of an inch in width) before it abruptly terminates. The meatus, as I have called the sack-like cavity which encloses the true acoustic sack or vesicle, is formed of pulpy membrane, and is apparently continuous with the corium of the whole body, but by dissection it can be separated entire. The acoustic vesicle is of various shapes, as we shall immediately see; but in all essential respects it is identical with the same part in the Lepadidæ; it is formed of the same peculiar, soft, elastic, brownish, transparent tissue, which seems to be composed of fine, transverse pillars, becoming towards the outside fibrous, and at their inner ends appearing when viewed vertically from above, like hyaline points. In _Coronula diadema_, I observed on the outside of the acoustic vesicle, some excessively minute bristles, only 1/3000ths of an inch in length, seated on little eminences. I examined carefully the contents of the vesicle in this species, in specimens well preserved in spirits, and there was nothing within but a very little, thin, pulpy fluid, and a few yellowish nucleated cells, here and there aggregated into small groups. In Coronula, the flattened acoustic vesicle is elongated, with a somewhat sinuous, but not very irregular margin (Pl. 27, fig. 4), and is without any ridges on the surface; its neck or orifice projects at right angles to the elongated portion, which stands obliquely to the tubular orifice of the meatus. In a moderately-sized specimen of _Coronula diadema_, the elongated portion of the acoustic vesicle was, 6/100ths of an inch in length. In Tubicinella, the acoustic vesicle is heart-shaped, with the neck attached to its broader end; and the surface is covered by zig-zag ridges. In _Balanus tintinnabulum_ (fig. 3), the acoustic vesicle is almost square at the lower end, with the neck placed at one of the upper corners; on the external surface, there is an oblique prominent ridge or fold, which sends off downwards another ridge; its length, in a large individual, was 5/100ths of an inch.