BODY-CAVITIES.—The existence of five separate body-cavities (Fig. 2) is one of the most fundamental facts in the anatomy of Balanoglossus. The first body-cavity, or cavity of the proboscis (_b.c_^1), is single and unpaired; the second body-cavities (_b.c_^2) are paired spaces, one belonging to each side of the collar; the third body-cavities (_b.c_^3) are similarly paired, and correspond with the trunk. While there is no connection between successive body-cavities, there are in certain regions communications between the two cavities of the same pair. Each of the paired cavities is at one time a closed lateral space between the skin and the alimentary canal. As the two spaces which constitute the pair grow towards one another, both above and below the alimentary canal, they come into such close apposition that they remain separated only by their conjoined walls. In this way are formed the dorsal and ventral mesenteries (Fig. 4, _d.m_, _v_), the former being the only one to persist in the higher Vertebrates. The body-cavities of the adult become to a large extent disguised by being traversed by connective tissue and muscles. {9}The hinder part of the proboscis-cavity is divided by the forward growth of the notochord (Fig. 3, _n_) into dorsal and ventral portions. The dorsal cavity in extending backwards becomes further subdivided into right and left halves, the latter typically opening dorsally to the exterior on the proboscis-stalk by an asymmetrical "proboscis-pore" (_p.p._). Two symmetrical proboscis-pores may, however, occur, or a median pore connected with the left division of the proboscis-cavity. These may be individual variations within the limits of a single species, or may occur as a normal feature of a species. [Illustration: FIG. 3.—Dorsal view of the anterior end of the body of _Dolichoglossus kowalevskii_, × 3. _c_, Collar; _c.n_, circular nerve; _c.p_, collar-pore; _d_, dorsal nerve; _g_, gill-pore; _n_, notochord; _n.s_, central nervous system, showing the anterior and posterior neuropores; _p_, proboscis; _p.p_, proboscis-pore; _p.s_, proboscis-stalk; _t_, trunk; _v_, ventral nerve. The nerve-plexus of the proboscis is represented as a black line. (After Bateson.)] The collar-cavities open by two "collar-pores" (Fig. 3, _c.p._), situated at the posterior end of the collar, into the first pair of gill-pouches, near their external opening. Willey has recently described[16] vestigial pores in relation with the "perihaemal spaces," a pair of dorsally situated outgrowths of the third body-cavities into the collar-region. Narrow "peripharyngeal spaces," also a forward growth of the third body-cavities, closely invest the pharynx in some species.