The trunk contains the alimentary canal, dorsal and ventral blood vessels, dorsal and ventral nerves, the gill-slits, the reproductive bodies, dorsal and ventral mesenteries and muscle bands. The nervous system consists of three parts: the central, located in the collar region, Fig. 5; the sub-epidermic network extending over the entire body just under the surface epithelium, Figs. 1-7; and the dorsal and ventral strands which are thickenings of the sub-epidermic network extending throughout the trunk, Figs. 1 and 7. There is also quite a decided thickening of the sub-epidermic network at the base of the proboscis, Figs. 5, 6. The vascular system consists of two parts, the central and the peripheral. The central is made up of the heart, a thin-walled vesicle at the base of the proboscis just dorsal to the notochord, and connected with it the proboscis gland, a plexus of capillaries just anterior to the notochord. Fig. 5. The peripheral system is composed of a ventral and a dorsal vessel. The dorsal starts at the heart and continues just ventral of the dorsal nerve throughout the length of the body. Figs. 1, 5, 7. The ventral vessel extends from the posterior border of the collar to the anal end. It is connected with the dorsal vessel by a circular vessel in the posterior edge of the collar. The mouth is situated ventrally at the base of the proboscis, within the collar, and opens directly into the straight alimentary canal. The latter is a straight tube extending from the mouth opening to the anus. Figs. 5, 1, 7, 9. The alimentary canal in the anterior part of the collar gives off a diverticulum, which grows forward and supports the proboscis. Because this diverticulum has the vacuolated appearance of the notochordal tissue of higher animals, it has been regarded as a notochord. It is largest at the base of the proboscis immediately anterior to the heart. Figs. 5, 6. The paired gill-slits occupy the region just posterior to the collar. They are arranged in two longitudinal grooves in the dorsal wall. The number increases throughout life, new slits appearing just behind those already in place. I found about twenty-five to be the average number, while particular individuals had as low as eighteen and twenty and as high as thirty and thirty-one. The gills are formed in the shape of a U. A skeletal rod or gill bar separates the gills from each other. The gills are supplied with blood from the dorsal vessel. Figs. 3, 7, 8. The sexes are distinct. The ovaries and testes are saccular organs arranged in a row along the gill and succeeding region. The sacs in other genera, for example Balanoglossus as described by Shipley, open directly on to the epidermis. I have been unable to see these openings in my preparations. Fig. 8 shows the position of the ovaries in the female; the testes in the male are in a similar location.