above drawing of these segments, but with the dorsal surface much flattened, in _Coronula diadema_; in most species of Balanus, however, the wedges formed by one segment being driven into another, are much sharper; on the other hand, in Xenobalanus they are nearly straight and transverse. The three posterior segments are always the most distinct; the two next segments are also distinct laterally, but along the dorsal surface they become, excepting in Xenobalanus and some few other cases, completely confluent. The greater distinctness of the posterior segments is conformable to what takes place in the higher Crustacea. The articulations between the segments are folded inwards, and are formed of thin membrane, which in some cases, as in _Coronula diadema_, forms a marked contrast with the much thicker, stiffer, and yellowish membrane of the segments themselves; in _Balanus tintinnabulum_, however, the whole membrane of the five thoracic segments is very thin, excepting small wedge-shaped portions along the medio-dorsal line. The infolded articulations between the segments supporting the three anterior pairs of cirri (at least in the Balaninæ), are much wider than those between the three posterior segments; the former segments, with their cirri, being consequently capable of being moved further apart from each other. Could there have been any doubt as to the distinctness and reality of the five thoracic segments, it would have been set aside by the arrangement of the muscles attached to them, as will presently be described. I need only add, that in many genera there are shield-like swellings at the exterior bases of the pedicels of the posterior cirri, which I for some time thought were the epimeral elements of the thoracic segments; but I now believe them to be parts of the pedicels of the cirri. The basi-exterior margin, moreover, of the pedicel of the third pair of cirri, in many species of the Balaninæ (Pl. 25, fig. 1), is produced as a plate, thickly fringed with fine hairs, half across the dorsal surface of the thorax; serving, apparently, as a brush to clean the sack, or to prevent the ingress of any intruding substance. [30] Von Siebold and Stannius, in their 'Anatomie Comparée,' tom. i, p. 473, and p. 440, (foot-note), consider the articulated probosciformed penis as an elongated abdomen; a view which, at the commencement of my examination, I was tempted to admit; but the position of the caudal appendages on the dorsal basis of the penis, suffice, I think, to show that this view is not correct; for these caudal appendages evidently correspond with those borne on the very extremity of the abdomen in the pupa. Nor, indeed, does the position of the anus accord well with such a view.