_Cephalodiscus_, though no more than two or three millimetres in length, is provided with practically all the important organs possessed by Balanoglossus. Its proboscis or "buccal shield" (Fig. 10, _ps_) is a large flattened structure, which overhangs and entirely conceals the mouth. The anterior body-cavity opens to the exterior by two symmetrically placed proboscis-pores (_p.p_), just in front of the tip of the notochord (_nch_). The collar, which has paired body-cavities, is produced dorsally into 4-6 {25}pairs of plume-like arms, which bear an immense number of pinnately-arranged tentacles. The arms, which may end in a swollen bulb,[46] have ventral grooves along which food doubtless travels to the mouth by ciliary currents. The anterior edge of the ventral half of the collar is drawn out into a narrow flap or operculum (Fig. 11, _op_), in front of which is the mouth, and behind it the gill-slits (_g_) and collar-pores (_c_). The central nervous system (_n.s_) is a thick mass of nerve-tissue in the dorsal epidermis of the collar; it is not sunk beneath the skin as in Balanoglossus. The details of the nervous and vascular systems, and the development of the buds, have been described by Masterman. In the dorsal region of the collar the alimentary canal has a slender diverticulum, the notochord, which passes into the base of the proboscis; it is believed by Masterman to have a function similar to that of the neural gland (cf. p. 52) of Tunicates. The next part of the alimentary canal, the pharynx,[47] has a single pair of simple gill-slits opening to the exterior immediately behind the collar-pores. The short oesophagus (Fig. 10, _oes_) is followed by the wide stomach (_st_), and this by the intestine (_int_), which opens by the anus (_a_) near the front end of the body. [Illustration: FIG. 11.—Longitudinal section through _Cephalodiscus dodecalophus_, passing through the two sides of the body; _a_, tentacular arm; _b.c^2_, collar-cavity; _b.c^3_, trunk-cavity; _c_, collar-pore; _g_, gill-slit; _i_, intestine; _n.s_, central nervous system; _o_, oesophagus; _op_, operculum; _p_, pharynx; _s_, stomach.]