BODY-WALL AND NERVOUS SYSTEM.—The body-wall (Fig. 4) consists externally of a thick ciliated epidermis (_e_), containing numerous gland-cells which secrete an abundant mucus. Beneath the epidermis is a basement-membrane, while more internally are layers of muscles, whose arrangement differs in different parts of the body and in different species. The nervous system consists of a plexus of cells and fibres which lie in the basal part of the epidermis of all parts of the animal, outside the basement-membrane; the thicker portions of the plexus forming definite nerve-tracts. This intimate connexion {10}between the epidermis and the nervous system is usually restricted to embryonic life in other animals. [Illustration: FIG. 4.—_Ptychodera bahamensis_, Bahama Is. Transverse section through the branchial region. _b_, Branchial part of pharynx; _b.c_^3, third body-cavity; _d.m_, dorsal mesentery; _d.n_, dorsal nerve; _d.v_, dorsal vessel; _e_, epidermis, with nerve-layer (black) at its base; _g_, genital wing; _g.p_, gill-pore, encroached on by the tongue-bar (_t_); _l_, lateral septum; _m_, longitudinal muscles; _o_, oesophageal or alimentary part of pharynx; _r_, reproductive organ; _t_, tongue-bar; _v_, ventral mesentery and ventral vessel; _v.n_, ventral nerve. (After Spengel.)] The main nerves of Balanoglossus are a dorsal and a ventral tract in the trunk region (Fig. 4, _d.n_, _v.n_), a circular tract (Fig. 3, _c.n_) connecting these two at the posterior edge of the collar, and a strong concentration of nerve-tissue round the whole of the proboscis-stalk, and of the posterior end of the proboscis (Fig. 3). In the region of the collar the nervous system attains its highest development, taking the form of a median cord passing above the alimentary canal. This cord, known as the central nervous system (Fig. 7, _n.s_), runs through the cavity of the collar, but is connected with the epidermis at each end. It thus becomes continuous in front with the nerve-layer on the proboscis-stalk, while posteriorly it passes into the dorsal and the circular nerve-tracts. In nearly all cases the epidermis is pushed into the cord at the points where it passes into the skin, in the form of an anterior and a posterior "neuropore" (Fig. 3). A transverse section through the extreme front or hind end of the collar accordingly shows a tubular nervous system. In certain species, as in _Glossobalanus sarniensis_ and _Ptychodera flava_, a central canal, opening in front and behind, exists throughout the entire length of the central nervous system, while in _G. minutus_ a canal of this kind occurs in the young animal, but not in the adult. The central nervous system is developed as a longitudinal {11}dorsal groove in the larva,[17] and in a similar manner in the collar which is formed as the result of regeneration after injury.[18] Balanoglossus is thus typically provided with a dorsal, tubular, central nervous system, and although this