_Sack_, _Body_, _Cirri_, _Mouth_.--A slit-like orifice between the opercular valves leads into the _sack_, in which the body is lodged. The body consists of the six (perhaps the seven) posterior thoracic segments of the archetype Crustacean; the first of these six segments (or first two, if there be seven segments) is developed on its dorsal aspect into a part, which I have called the _prosoma_[4] (see fig. 1, _c_, Pl. 25). There is no abdomen. The thoracic segments support six pairs of _cirri_. Each cirrus consists of a two-jointed _pedicel_, carrying two multiarticulated _rami_. Rarely there are articulated _caudal appendages_ (_appendices caudales_) on each side of the anus. The prominent mouth consists of a _labrum_, _palpi_, _mandibles_, _maxillæ_, and _outer maxillæ_, the latter resembling a lower lip: these organs may be conveniently spoken of, after Milne Edwards, as _gnathites_. Within the sack, attached to its carino-lateral end, a folded membrane forms the _branchiæ_. The sheets of ova lying within the sack are called the _ovigerous lamellæ_. [4] A discussion on the homologies of the different parts is given under the head of the Metamorphoses of the Balanidæ. I have often found it convenient to designate the membrane investing the body, lining the sack, &c., by its proper chemical name of _chitine_, instead of by horny, or other such equivalents; but when covering parts of the shell, for brevity's sake I have often spoken of it as an _epidermis_, but I do not believe that such is its nature. When this membrane sends into the body of the animal rigid projections or crests, for the attachment of muscles or any other purpose, I call them, after Audouin, _apodemes_. For the underlying true skin, I use the term _corium_.