The true mouth is a small pore at the bottom of a large vestibule (the stomodaeum), placed at the anterior end of the ventral surface (Figs. 70 and 71), and formed by the "oral hood," which may be a prolongation forwards of the atrial or metapleural folds at each side. The edges of the oral hood bear 12 to 20 pairs of cirri (Fig. 70, _cir_) or ciliated tentacles (strengthened by skeletal rods), which form a sensory fringe around the opening. The anus (Figs. 70 and 71, _an_), is asymmetrical, being {117}placed on the left side of the ventral fin, some distance behind the atriopore, and not far from the posterior end of the body. The short region behind the anus and surrounded by the caudal fin may properly be called "tail." The current of water for respiratory and nutritive purposes, and which may carry the ova and spermatozoa to the exterior, usually passes in at the mouth and out at the atriopore, as in the Tunicata. On occasions, however, it is said to be reversed. GENERAL STRUCTURE.—The general plan of organisation of the body (see Fig. 71) is that a longitudinal skeletal axis, the notochord (_nch_), separates a dorsal nervous system (_sp.cd_) from a ventral reduced coelom (_coel_), in which lie the alimentary canal (_int_), the gonads (_gon_), and other organs. Thus a transverse section of the body (see Fig. 72) shows the typical Chordate arrangement of parts, and is comparable with a transverse section of a tadpole, a young fish, or a larval Ascidian. A peribranchial (_atr_) or atrial cavity (which is morphologically a part of the external world shut in) lies external to the coelom and body-wall around the pharynx and the greater part of the alimentary canal, and opens to the exterior by the atriopore. As in the Tunicata, the perforations (gill-slits) in the wall of the pharynx (_br.cl_) open into the atrial cavity and so indirectly to the exterior. MUSCULATURE.—The thick body-wall is largely formed by muscular tissue metamerically segmented into about 60 myotomes (Fig. 71, _myom_). These muscle-masses, which (as is usual in Vertebrata) are thickest dorsally at the sides of the notochord and spinal chord (Fig. 72, _m_), are so arranged as to present the appearance in a lateral view of the body of a series of shallow cones (<<) fitting into one another and with their apices directed forwards. The muscle fibres are striated, and run longitudinally along the body from the anterior to the posterior edge of each myotome, so as to be attached at their ends to the two septa of connective tissue which form the boundaries of the myotomes. These septa, the myocommas, are conspicuous features in the external appearance of the body (Fig. 70, B). They are not arranged so as to be opposite one another on the two sides, but the myotomes on the right and left sides alternate, as can be seen in a transverse section (Fig. 74, A, p. 121).