Annelida (Lat. annellus, a small ring), red-blooded worms, such as the earth worm, the lug worm, and the leech. They are the only section of invertebrate animals which have red blood. They form an extensive class, subdivided into four orders by Milne-Edwards. The body has an elongated form, with distinct, soft, semi-cartilaginous annulations, connected together by longitudinal oblique muscles, enabling the animals to twist themselves in various directions. The whole is covered with a moist skin, indicating by slight segments the soft annuli beneath. The first segment is furnished with a mouth, and in some species with eyes and tentacles; the last segment is furnished in some cases with bristle-like appendages, and in others, as in the leech, it is dilated into a sucker. Each segment has usually minute setae), or spines, which are useful in locomotion. In some species vascular tufts are observed, which serve as respiratory organs. There is a system of veins and arteries. The nervous system consists of ganglia, united by means of a double nervous cord. Each individual is bisexual. The common or earth worms seek safety by retiring into holes which they bore in soft earth, mud, or sand.

The sabella and terebella of the seashore agglutinate around them particles of sand and of broken shells to form a case in which they dwell. The serpula exudes a calcareous secretion to form a long twisted tube, in which the animal resides, and from which it protrudes its head and respiratory tufts. The four orders of this class are: 1, the dorsibranchiata, or er-rantes, including the sea centipedes and sea mice; 2, the tubicolae, which include those that live in tubes, as the serpula; 3, the ter-ricolae, including the common earth worm; and 4, the suctoriae, having suctorial disks, as the leech. From the recent researches of Prof. Edward S. Morse, it appears that the brachiopoda come near the tubiculous annelids; and he therefore removes them from the mollusca to the annelid division of articulata.