Underneath the axial rod or chorda runs a very simple alimentary canal, a tube that opens on the ventral side of the animal by a mouth in front and anus behind. The oval mouth is surrounded by a ring of cartilage, on which there are twenty to thirty cartilaginous threads (organs of touch, Figure 2.210 a). The alimentary canal divides into sections of about equal length by a constriction in the middle. The fore section, or head-gut, serves for respiration; the hind section, or trunk-gut, for digestion. The limit of the two alimentary regions is also the limit of the two parts of the body, the head and the trunk. The head-gut or branchial gut forms a broad gill-crate, the grilled wall of which is pierced by numbers of gill-clefts (Figure 2.210 d). The fine bars of the gill-crate between the clefts are strengthened with firm parallel rods, and these are connected in pairs by cross-rods. The water that enters the mouth of the Amphioxus passes through these clefts into the large surrounding branchial cavity or atrium, and then pours out behind through a hole in it, the respiratory pore (porus branchialis, Figure 2.210 c). Below, on the ventral side of the gill-crate, there is in the middle line a ciliated groove with a glandular wall (the hypobranchial groove), which is also found in the Ascidia and the larvae of the Cyclostoma. It is interesting because the thyroid gland in the larynx of the higher vertebrates (underneath the "Adam's apple") has been developed from it. (FIGURE 2.212. Transverse section of an Amphioxus-larva, with five gill-clefts, through the middle of the body. FIGURE 2.213. Diagram of the preceding. (From Hatschek.) A epidermis, B medullary tube, C chorda, C1 inner chorda-sheath, D visceral epithelium, E sub-intestinal vein. 1 cutis, 2 muscle-plate (myotome), 3 skeletal plate (sclerotome), 4 coeloseptum (partition between dorsal and ventral coeloma), 5 skin-fibre layer, 6 gut-fibre layer, I myocoel (dorsal body-cavity), II splanchnocoel (ventral body-cavity).) Behind the respiratory part of the gut we have the digestive section, the trunk or liver (hepatic) gut. The small particles that the Amphioxus takes in with the water--infusoria, diatoms, particles of decomposed plants and animals, etc.--pass from the gill-crate into the digestive part of the canal, and are used up as food. From a somewhat enlarged portion, that corresponds to the stomach (Figure 2.210 e), a long, pouch-like blind sac proceeds straight forward (f); it lies underneath on the left side of the gill-crate, and ends blindly about the middle of it. This is the liver of the Amphioxus, the simplest kind of liver that we meet in any vertebrate. In man also the liver develops, as we shall see, in the shape of a pouch-like blind sac, that forms out of the alimentary canal behind the stomach.