The system consists of the following parts:--A dorsal vessel applied to the alimentary canal is continued anteriorly into a median vessel, which traverses the proboscis to its tip. Here the vessel splits, and each half returns along the lateral edge of the proboscis; they reunite around the oesophagus and form a single ventral vessel, which lies above the ventral nerve-cord. The ventral vessel, which ends solidly behind, sends off a branch which forms a ring around the intestine and opens into the posterior extremity of the dorsal vessel. In _Echiurus_ and _Thalassema_ the same vessel forms a ring round a stout muscle, which connects the bases of the two ventral setae before passing to surround the intestine. Amoeboid corpuscles float in the fluid contents. The nephridia vary in number from a single one in _Bonellia_ to three pairs in many species of _Thalassema_. Their external openings are ventral, and on the same level as the ciliated funnel-shaped nephrostomes. The posterior wall of the organ is produced into a long blind sac, which is lined by secretory cells. The nervous system is a single ventral cord, which starts from a circumoesophageal ring. This ring is involved in the growth of the proboscis, and is drawn out with it. Thus there is a lateral nerve near each edge of the proboscis which unites with its fellow dorsally above the oesophagus at the tip of the proboscis, and ventrally beneath the oesophagus, where they fuse to form the ventral nerve-cord. There are no specialized ganglia, but ganglion-cells are scattered uniformly along the nerve-cords. The ventral cord gives off rings, which run into the skin at regular intervals. The reproductive cells are modified coelomic cells, which lie on the ventral vessel. They escape into the coelomic fluid and there develop. When mature they leave the body through the nephridia. _Bonellia_ and _Hamingia_ are very interesting examples of sexual dimorphism. The female has the normal Echiuroid structure, but the male is reduced to a minute, flattened, planarian-like organism, which passes its life usually in the company of two or three others in a special recess of the nephridia of the female. Its structure may be gathered by a reference to fig. 3. [Illustration: FIG. 3.--Adult male, _Bonellia viridis_, Rol. The original was 1.5 mm. long. The nervous system is not shown. (After Selenka.) a, Generative pore with spermatozoa coming out. b, Anterior blind end of intestine attached to the parenchymatous tissue by muscular strands. c, Green wandering cells containing chlorophyll. d, Parenchymatous connective tissue. e, Epidermis. i, Intestine. j, Vas deferens. l, Internal opening of vas deferens. m, The left anal vesicle. n, Spermatozoa in the body-cavity.]