Fig.214. Transverse section of a young Amphioxus, immediately after metamorphosis. Fig. 215. Diagram of preceding. Fig. 214—Transverse section of a young Amphioxus, immediately after metamorphosis, through the hindermost third (between the atrium-cavity and the anus). Fig. 215—Diagram of preceding. (From _Hatschek._) _A_ epidermis, _B_ medullary tube, _C_ chorda, _ D_ aorta, _E_ visceral epithelium, _F_ subintestinal vein. _1_ corium-plate, _2_ muscle-plate, _3_ fascie-plate, _4_ outer chorda-sheath, _5_ myoseptum, _ 6_ skin-fibre plate, _7_ gut-fibre plate, _I_ myocœl, _II_ splanchnocœl, _I_1 dorsal fin, _I_2 anus-fin.) These important structures are seen very clearly in the trunk of the amphioxus (the latter third, Figs. 212–215), but it is otherwise in the head, the foremost third (Fig. 216). Here we find a number of complicated structures that cannot be understood until we have studied them on the embryological side in the next chapter (cf. Fig. 81). The branchial gut lies free in a spacious cavity filled with water, which was wrongly thought formerly to be the body-cavity (Fig. 216 _A_). As a matter of fact, this atrium (commonly called the peribranchial cavity) is a secondary structure formed by the development of a couple of lateral mantle-folds or gill-covers (_M_1, _U_). The real body-cavity (_Lh_) is very narrow and entirely closed, lined with epithelium. The peribranchial cavity (_A_) is full of water, and its walls are lined with the skin-sense layer; it opens outwards in the rear through the respiratory pore (Fig. 210 _ c_). On the inner surface of these mantle-folds (_M_1), in the ventral half of the wide mantle cavity (atrium), we find the sex-organs of the Amphioxus. At each side of the branchial gut there are between twenty and thirty roundish four-cornered sacs, which can clearly be seen from without with the naked eye, as they shine through the thin transparent body-wall. These sacs are the sexual glands they are the same size and shape in both sexes, only differing in contents. In the female they contain a quantity of simple ova (Fig. 219 _g_); in the male a number of much smaller cells that change into mobile ciliated cells (sperm-cells). Both sacs lie on the inner wall of the atrium, and have no special outlets. When the ova of the female and the sperm of the male are ripe, they fall into the atrium, pass through the gill-clefts into the fore-gut, and are ejected through the mouth.