In my former volume on the Lepadidæ, I have described the _ovigerous fræna_ occurring on the two sides of the sack, to which the _ovigerous lamellæ_ are attached by a peculiar glandular secretion: in the Balanidæ there are no ovigerous fræna, but the branchiæ just described are identical with the fræna in essential structure and in position; differing only in being placed a little nearer to the carinal end of the sack, and in being generally (but not always) larger and more plicated: seeing this, and that in _Alcippe lampas_, and in some species of Pollicipes,--the genus which comes nearest to the Balanidæ,--the ovigerous fræna are large and are destitute of glands, and have therefore lost their normal function of supporting the ovigerous lamellæ, I can hardly doubt that the _branchiæ_ in the Balanidæ are the _ovigerous fræna_ of the Lepadidæ in a modified condition; a transformation of function not greater than that of the swimming bladder of a fish into the lungs of the higher Vertebrata.[29] [29] There is, I conceive, no foundation for the belief of some authors that the branchiæ of the Balanidæ are in any way connected with the ovaria. _Thorax and Body._ _Parts of the body included within the shell or carapace._--These parts (Pl. 25, fig. 1) consist of the prominent mouth, and of the thorax (_c′_), with its largely developed portion, called the prosoma (_c_), and with its appendages. The abdomen is quite rudimentary, being represented merely by a small portion of membrane surrounding the anus, and sometimes inserted like a wedge between the inwardly inflected posterior thoracic segments; in only two genera (Catophragmus and Pachylasma), its nature is rendered somewhat plainer by supporting caudal appendages. The probosciformed penis lies folded under the thorax; and I believe (from what is seen in the anomalous genus Proteolepas), that it normally arises from the ventral surface of the terminal point of the rudimentary abdomen.[30] The thorax is laterally compressed, the ventral surface being very narrow, with the bases of the cirri placed closely together. It consists, in appearance, of two very different portions; one a soft, more or less rounded bag, which I have called the prosoma; and the other, which supports the five posterior pairs of cirri, is narrower, invested with stiffer membrane, and is more or less distinctly composed of five segments. These segments (Pl. 26, fig. 8) on their dorsal and dorso-lateral surfaces, are generally driven like wedges one into the other, with their points directed anteriorly: on the ventral surface the articulations are transverse. The prolongation (_e_) of the thin membrane (_a_) surrounding the anus (_b_), that is, the rudiment of the abdomen, which sometimes carries caudal appendages, almost divides (in appearance, whether really I know not) the hindermost thoracic segment along the medio-dorsal line, into two parts. I have given the