COELOM.—In the young larva there are at first (as in Balanoglossus) five coelomic spaces, a median anterior "head-cavity," a pair of antero-lateral "collar-cavities," and a pair of more posterior long lateral grooves from which arise, in the later larva, the segmented myotomes and ventrally a large coelomic space surrounding the alimentary canal and separating it from the body-wall. In the adult animal, however, the coelom has been so much displaced by the formation of the spacious atrium that in front of the atriopore it can only be recognised as a series of canals and crevices. The relations of coelom to atrium in the region of the intestine are {124}seen in Fig. 74, B, and in the region of the pharynx in Fig. 74, A. Fig. 72 shows the distribution of the spaces more in detail (see also Fig. 71). Beginning anteriorly, along the dorsal surface of the pharynx and beneath the notochord run a pair of dorsal coelomic canals, one at each side of the epipharyngeal groove; these give off ventral diverticula which pass down the primary branchial bars of the pharyngeal wall and unite ventrally in a median tube, the endostylar coelom (see Fig. 72, _ec_). At the posterior end of the pharynx these dorsal and ventral canals unite in a narrow coelomic space encircling the stomach, inside the wall of the atrium, and sending an extension forwards around the liver (Fig. 74, A, _l_). In the region of the intestine, behind the atriopore, the coelom is allowed to expand to its primitive condition on the left-hand side (Fig. 74, B), but is still reduced on the right side, where there is a prolongation of the atrial cavity reaching nearly to the anus. All these coelomic spaces are lined by a coelomic epithelium.