The most conspicuous organ of these primitive worms is the proboscis, a hollow string which is shot out with great speed and force from the front end of the usually cylindrical body. At the end of the string, in several groups, is a sharp-pointed, barbed spear-like stylet with which the prey, usually some minute water animal, is speared. The victim then is drawn back into the mouth by the attached hollow thread. Some groups have no stylets. The thread, upon which is a mucilage-like mucous, is used like a lassoo and coiled tightly around the prey. The proboscis is associated so closely with the brain that, like the retina of the eye, it has sometimes been considered an extension of it. The thread often is as long as the worm itself. It is shot out with such force that is frequently breaks off and continues to lead an independent life for a few hours. A new proboscis always develops. When coiled, the proboscis rests in the center of the two-lobed brain. It is continuously shot out and pulled in and probes the water around it. Presumably at first it was an extremely sensitive sensory organ by which the brain was kept aware of its surroundings. The attached stylet, an offensive weapon, was a later development. In a few cases the thread carries a multitude of unattached barbed points, a sort of machine-gun arrangement, which can be hurled in all directions in the hope of hitting something. It also carries tiny hooks by which it can be attached to some object. By means of the attached line the worm pulls itself forward over beach or sea bottom, its ordinary means of locomotion. It is also able, however, to glide like a planarian and to swim. Nemertinea breath through the walls of the oesophagus, or gullet. When the tide comes in, shore-dwelling species rapidly swallow and eject mouthfuls of salt water. Oxygen to purify the blood is obtained from the water. The blood circulates in two or three vessels. It is a colorless plasma in which float both green and red corpuscles.