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EOHIUROIDEA 625 (H. L. Jameson, Zool. Jahrb. Anat., 1899). The alimentary oesophagus and form a single ventral vessel, which lies canal is divisible into fore-gut, mid-gut, and hind-gut, and above the ventral nerve-cord. The ventral vessel, which the first named can be further divided into pharynx, oeso- ends solidly behind, sends off a branch which forms a ring phagus, gizzard, and crop, mainly on histological grounds. around the intestine and opens into the posterior exThe mid-gut is characterized by the presence of a ciliated tremity of the dorsal vessel. In Echiurus and Thalassema groove, from which arises the collateral intestine or siphon, the same vessel forms a ring round a stout muscle, which a second tube which rejoins the alimentary canal lower connects the bases of the two ventral setae before passing down. Similar collateral intestines are familiar in the to surround the intestine. Amoeboid corpuscles float in Echinids and certain Polychsets (Capitellidm). The rec- the fluid contents. The nephridia vary in number from a tum receives the openings of a pair of very characteristic single one in Bonellia to three pairs in many species of organs, the anal vesicles. Each consists of a branching Thalassema. Their external openings are ventral, and on tube, the tips of whose twigs terminate in minute ciliated the same level as the ciliated funnel-shaped nephrostomes. funnels. The anal vesicles are thought to be excretory; The posterior wall of the organ is produced into a long whether this be so or not, they undoubtedly have some 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 in 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. Larva.—The larva is a typical trochosphere, which, although of' a temporary character, shows a distinct segmentation of the mesoblast, of the nervous system, and of the ciliated and pigmented structures in the skin, resembling that of Chaetopods. The preoral lobe persists as the proboscis. The sexes of the larvae are not determinable in the early stages, but when a certain growth has been reached in Bonellia the males seek the proboscis of the adult females, and passing into the mouth undergo there the transformation into the planarian-like parasite which is the fully-formed male. This now creeps along the body of the female and takes up its home in her nephridia. Classification and Distribution.—The Echiuroidea consists of the following genera(1) Bonellia (Rol.), with four species, widely distributed, but inhabiting the temperate and warmer waters of each hemisphere. (2) EchiuPig. 2.—Female Bonellia viridis, Rol. Opened along the left side. X 2. f> short;intestine b, bristle; dpassing through the mouth into rus (Guerin-Meneville), with four species. This genus theproboscis pharynx;cut c, coiled f anal tufts or vesicles ; e, ventral nerve cord ; /, ovary borne on ventral vessel running parallel with e ; reaches from the Arctic waters of both hemispheres into g, position of anus; A, position of external opening of nephridium * the cooler temperate regions. (3) Hamingia (Kor. and nephridium—the line points towards, but does not reach, the internal opening. Dan.), with one species, which has been taken in the Arctic Sea and the Hardanger Fjord. (4) Saccosoma influence on the amount of fluid found in the coelom. (Ivor, and Dan.) was described from a single specimen The ccelomic fluid contains as a rule both amceboid and dredged about half-way between Iceland and Norway. rounded corpuscles, and, when ripe, the products of the ip) Thalassema (Gaertner, Lamarck), with twenty-one gonads. 'A closed system of vessels, usually called the species. This genus is in the main a denizen of the warmer vascular system, is present. There are, however, no capil- waters of the globe. Sixteen species are found only in laries connected with this, and it is confined to certain tropical or subtropical seas, three species are Mediterportions of the body. It can possess few of the functions ranean (Mt. Stat. Neapel, 1899), whilst three species are usually associated with a vascular system, and its main from the eastern Atlantic, where the temperature is modiuse is probably to assist in the expansion of the proboscis. fied by the Gulf Stream (Shipley; see Willey’s Zoological The system consists of the following parts :—A dorsal vessel Results, part iii., 1899; Proc. Zool. Soc. Bond., 1898, 1899 ; applied to the alimentary canal is continued anteriorly into and Cambridge Natural History, ii.). The following are a median vessel, which traverses the proboscis to its tip. found in the British area :—E. pallasii (Guerin-MeneHere the vessel splits, and each half returns along the ville), Th. neptuni (Gaertner), and Th. lankesteri (Herdlateral edge of the proboscis; they reunite around the man, Q.J.M.S., 1898). S. HI.— 79