In our two commonest, tidal, sessile Cirripedes, viz. _Balanus balanoides_ and _Chthamalus stellatus_, I have observed that, when left uncovered by water, they kept the orifice of their operculums a little open, with a bubble of air within their sacks, so that the orifice was in fact closed by a thin septum of water, with air beneath; when disturbed, they closed their operculums with force, and expelled the bubble of air with a clicking noise, which has been noticed by Dr. Coldstream,[27] and has been thought to be made by the movement of the operculum itself. _Bal. crenatus_, a deep-water species, when out of water, keeps its operculum closed. [27] 'Encyclopædia of Anatomy and Physiology;' article Cirrhopoda. In Coronula, Platylepas, Tubicinella, and Xenobalanus, each branchia[28] consists of two unequal folds, both plicated on both sides: in the two latter genera, they extend far down the deep and elongated sacks, and hence the area of surface altogether gained is extremely great. In most of the species of Chthamalus, the branchia consist of a small fillet barely plicated: in the allied _Chamæsipho columna_, they are rudimentary, forming a smooth little pouch only 1/100th of an inch in length: in _Chthamalus scabrosus_ they are quite aborted, being perhaps represented by a slight hairy ridge; but in _Chthamalus dentatus_, and therefore within the limits of the same genus, the branchiæ (and this seems to me a singular fact) are large, each being composed of two plicated folds, as in Coronula. Tapering filaments situated near the bases of the cirri, such as those occurring in several species of the Lepadidæ, are not found in any sessile Cirripede; but I have observed nearly similar filaments, projecting upwards and inwards at the base of the sack, in several species of Balanus and in Coronula; those which I examined were simply occupied by delicate corium, and no doubt must aid in exposing a greater surface of corium to the circumambient water. [28] Burmeister has given a good figure (Tab. 2, fig. 10) of the branchiæ of Coronula, (but the two folds are shaded too unequally), in his 'Beiträge zur Naturgeschichte der Rankenfüsser.'