The movements of the bristly feet of the Errantia are reflex, depending on the nerve-centres in their segments; but they are controlled and connected by the double cord which passes through them. Every hair, cirrus, and tentacle on the bodies of the Errant Worms is a living organ of feeling, shrinking at the smallest touch, but enabling them to select their food, to move towards and retreat from objects, and to thread their way through the most intricate labyrinths with unerring certainty, which seems to render them independent of eyes; yet many of them have multitudes of eyes, or rather eye-specks, according to the genera. Some have but one eye-speck placed in the forehead; one genus has a double row throughout their whole length, two in each segment, while the Amphicora has two in its tail. All these eye-specks have their crystalline lens, pigment-layer, and nerve-bulb, so that the Errant Worms must see objects, and their motions show that they do; but we can form no idea of the kind of vision. Besides the variety of organs on the skin of the Errantia, some of these worms have two rows of flat plates on their backs overlapping each other at their edges like the scales of a fish. They are well developed in the Aphrodita hystrix, or the Sea Mouse of fishermen, and its congeners. That Annelid, which is an inhabitant of European coasts, is thicker and broader than other sea-worms. The two rows of overlapping shields on its back, and the quantity of iridescent hairs, cirri, and other appendages covering the body, is so great as to form a kind of felt or fur like the skin of a mouse. The members of this genus of sea-worms have no gills properly so called; the only external sign of respiration is a periodical elevation and depression of the shields on their backs by the action of a complex system of muscles. The thick covering of felt on the body of the worm below the shields becomes filled with water during their elevation, which is ejected forcibly at the posterior end of the body during their depression. Although the water does not penetrate the thin skin on the back of the worm, its oxygen does, and is accumulated in the colourless liquid in which the stomach floats; and from it the blood, which is of a pale yellow colour, receives its oxygen. The feet of the worm are fan-shaped groups of sharp glassy bristles enclosed between two plates, which prevent them from hurting the animal when it puts them out or draws them in. The Aphrodita is male and female: the eggs escape through pores in the female, and are received in a kind of pouch beneath the dorsal shields till hatched. The embryo is an oval locomotive mass, with groups of cilia, and indications of an eye-speck: after swimming about for twenty-four hours, the segments begin to be developed.