arrangement does not extend beyond the limits of the collar, it shows a noteworthy resemblance to Vertebrate animals. In some cases the central nervous system is connected with the dorsal epidermis by a varying number (1-17) of median "roots," which have been compared by Bateson with the dorsal roots of the spinal nerves of Amphioxus, and are probably remains of the embryonic connexion of the collar nervous system with the dorsal epidermis. ALIMENTARY CANAL.—The mouth (Fig. 7, _m_) leads widely into the alimentary canal, which, passing through the collar, enters the branchial region, where it is characterised by the existence of communications with the exterior. These, the gill-slits, are developed, as in Vertebrates, as paired outgrowths of the alimentary canal, and new gill-slits are constantly being formed at the posterior end of the branchial region with advancing age. The maximum number of the gill-slits, and the extent of the branchial region, are by no means uniform throughout the Enteropneusta. Thus _Dolichoglossus otagoensis_ is said to have no more than 12 pairs, _Glossobalanus minutus_ only 40 pairs, while _Balanoglossus aurantiacus_ may have as many as 700 pairs. In _Ptychodera flava_ the variation is so great that Willey distinguishes[19] two extreme conditions as "macrobranchiate" and "brachybranchiate" respectively, although intermediate conditions are also found. It should be noted that Balanoglossus agrees with Amphioxus in the indefinite number of the gill-slits. The gill-slits usually have the form of the so-called "branchial pouches" or "gill-sacs" (Figs. 5, 6, _g.s_). Each ordinarily opens to the exterior by a small pore (Fig. 1, D, 5, _g.p_) or slit, situated on the dorsal side, in a shallow longitudinal groove not far from the middle line. The gill-sac has a complete wall of its own, and lies between the alimentary canal and the body-wall, communicating with the former by a U-shaped slit. While a dorsal {12}view of the animal thus shows a linear series of simple pores, a view of the pharynx from the inside appears as in Fig. 5. At the hind end of the pharynx the inner opening of the developing gill-sac is circular. Slightly further forward the dorsal side of the pore is indented into a crescent, which grows longer in a dorso-ventral direction, and becomes a U, whose two limbs are nearly separated by a mass of tissue, the so-called "tongue-bar" (Fig. 5, _t_). The special interest of this mode of development is that it is identical with what occurs in Amphioxus (p. 120), which is universally admitted to belong to the Chordata.