The Errant Worms have a distinct small head with a mouth, or rather an orifice, on the upper side of it, through which a cylindrical gullet is from time to time turned inside out, forming a kind of pear-shaped bag, whose surface is studded with secreting glands; and its extremity, which is perforated, is surrounded by a muscle that contracts strongly on whatever it is applied to, and holds it firmly while the re-inversion of the sac draws it into the body to be digested. This apparatus is unarmed in the genera Arenicola, Phyllodoce, and others, but in the Nereis it has one pair of strong curved horny jaws. In the Eunice there are three toothed jaws on one side and four jaws on the other side of the gullet, each pair having a different form, and the tiny Lombrinereis has eight little black hooks which are seen through its pellucid tissues, snapping like so many pairs of hooked scissors. The Errant Worms are voraciously carnivorous, and when the gullet is turned inside out the toothed jaws project, seize the prey, and drag it into a ciliated alimentary canal, for there is no proper stomach in these worms. The canal is generally straight, and terminates in a vent at the posterior end of the body. The respiratory organs of the Errantia are external gills of great variety of forms: they are chiefly like branching trees, or filamentary bushes, traversed by capillary bloodvessels. They are sometimes small, and arranged on every segment along both sides of the back; sometimes they are large and fixed only at intervals. Like the lower Annelids, they have two liquid systems, one red and the other colourless, and the circulation of the blood is the same; but as the pulsations of the vessel behind the head are too feeble to send the blood through the labyrinth of capillary vessels in these long worms, there is a supplementary heart, or pulsating vessel, in each segment of the worm, which partakes in and facilitates the general circulation. The Eunice and other very long worms may have hundreds of these centres of propulsion, which make the circulation rapid; and it is increased by the restlessness and activity of the worms themselves, which bring their gills perpetually into new strata of water. The nervous system of the Errantia consists of a double cord extending along the ventral side of the body, and united at equal intervals by double nerve-centres, as in fig. 131; but in the Annelids the two cords diverge below the gullet, surround it, unite again above that tube, and form a principal bilobed nerve-centre or brain. Each segment of the worm is occupied by a small double nerve-centre. In some of these marine worms there are hundreds of segments and as many nerve-centres. There are more than a thousand of these pairs of nerve-centres on the ventral cord of the Nemertes gigas, or Great Band Worm, which is sometimes forty feet long and an inch broad. The head is like a snake, and the bristled feet are jointed to enable it to move over hard surfaces.