ECHIUROIDEA (Gr. [Greek: echis], adder, and [Greek: oura], tail), the zoological name for a small group of marine animals which show in their larval life-history a certain degree of segmentation, and are therefore grouped by some authorities as Annelids. Formerly, together with the Sipunculoidea and Priapuloidea, they made up the class Gephyrea, but on the ground that they retain in the adult a large preoral lobe (the proboscis), that they have anal vesicles, that their anus is terminal, that setae are found, and finally that they are segmented in the larval stage, they have been removed from the class, which by the proposed further separation of the Priapuloidea on account of their unique renal and reproductive organs, has practically ceased to exist. [Illustration: FIG. 1.--A, _Bonellia viridis_, Rol., [female]; B, B. _fuliginosa_. Both natural size. a, grooved proboscis; b, mouth; c, ventral hooks; d, anus.] Echiuroids are animals of moderate size, varying roughly from one to six or seven centimetres in length, exclusive of the proboscis. This organ is capable of very considerable extension, and may attain a length in _Bonellia viridis_ of about a metre and a half (fig. 1). It is grooved ventrally and ciliated. At its attachment to the body the groove sinks into the mouth. In _Bonellia_ the proboscis is forked at its free end, but in the other genera it is short and unforked. The body is somewhat sausage-shaped, with the anus at the posterior extremity, surrounded in _Echiurus_ by a single or double ring of setae. The skin is usually wrinkled, and in _B. viridis_, _Thalassema lankesteri_, _Th. baronii_, _Hamingia arctica_, and in the larva of many species, is of a lively green colour. A pair of curved bristles, formed in true setal sacs as in Chaetopoda, project from the body a short distance behind the mouth, and are moved by special muscles; they are of use in helping the animal to move slowly about, and they take a large share in the burrowing movements (C.B. Wilson, _Biol. Bull._, 1900), for some species tunnel in the mud and sand and form more or less permanent burrows, the walls of which are strengthened by mucus secreted from the skin. The openings of the burrows become silted up, leaving, however, a small aperture through which the proboscis is extruded. This organ carefully searches the neighbourhood for particles of food. When these are found the grooved proboscis folds its walls inwards, and the cilia pass the particles down the tube thus formed to the mouth. Echiuroids also move by extending the proboscis, which takes hold of some fixed object, and, then contracting, draws the body forwards. Recently it has been shown that _Echiurus_ swims freely at night-time, using for locomotion both the proboscis and the contraction of the muscles of its body-wall. The motion is described as "gyratory," and the anterior end is always carried foremost. Those species which do not burrow usually conceal themselves in crevices of