PCUIATORY SYSTEM.
The nervous system consists of a pair of gang lia lying above the oesophagus, known as the brain, from which the nerve trunks arise. Usual ly there are two such trunks, which pass down ward and backward around the (esophagus, meet ing in the mid-ventral line and running backward to the rear of the body as a double cord. On this there are ganglia in each segment. The sense of touch is usually acute in annelids, and is often localized in tentacles and papilla. Many species have eyes more or less highly organized; some have sensory pits, supposed to be smelling organs; some have sensory papilla, which from their occurrence around the mouth are supposed to he organs of taste; and a very few have oto cysts, or positional organs. In all annelids, ex cept a few aberrant forms, excretion takes place. by means of nephridia, and these are usually ar ranged a pair in each segment. These nephridia are coiled tubes, one end widened to form a fun nel and opening in the body cavity, and the other opening to the exterior. See NERVOUS SYSTEM.
Respiration is either by gills, which are of very various structure and appearance, or through the surface of the body or some part of the alimentary canal. The latter varies greatly with the habits of the worms, but the anal open ing is always at the posterior end of the body. The muscular system is usually well developed, for many of these worms are very active ani mals. The sexes are generally separate, but many annelids are hermaphrodites. Nearly all lay eggs, and these are sometimes provided with a shell. See RESPIRATORY SYSTEM GILLS; Mus cui,An SYSTEM.
Annelids are widely distributed over the world; while the majority are marine, a large number are found in fresh water or in the earth. .Many are carnivorous, hut some are almost whol ly vegetable feeders. Some are sluggish, but the majority are active, and some move with remarkable rapidity. They vary'greatly in size, some being almost microscopic. while others are several feet long. They are usually dull colored, but some, especially tropical species, are gorgeously arrayed. Aside from the part they play in the economy of natme as soil producers and scavengers, they are of little use to man.
Leeches were formerly (and are still sometimes) used in medicine for blood-letting, and a few species are used as food by savages, notably the palolo-wo•m (q.v.).
The classification of the annelids has always been a matter of great difficulty, as there are several other groups to which they seem to be related or which they superficially resemble. The matter is not definitely settled, but it seems best now to regard them as a phylum, or type, co ordinate with Mollusea, Arthropoda, etc., and containing two well-marked classes, and two others whose relationships are very obscure. The largest and most important of these classes is that of the Chwtopoda, in which the blood system is closed and the external rings, of the body cor respond to the internal segments. They have loeo motive organs in the form of seta% or appendages provided with them. The class includes a very great number of species of widely different struc ture and appearance, and the most convenient, though possibly not the most natural way to di vide mt is into three groups, Polychwta, Oligochm ta, and Myzostomida. The last named are a very small group of curious, degenerate annelids which live parasitically on erinoids. The body is fiat and unsegmented, and has neither circula tory nor excretory system. The second class is Gcphyrea, containing marine Annulata "devoid of any trace of segmentation in the adult con dition, without pars podia, and either without setae, or with only a limited number." It in cludes Sipunculus, Echiurus, and a few closely related forms. The third class is .1rehi-annelida, minute marine worms, faintly segmented, and representel by only two families—the Ilistrio drilidw, parasitic on lobsters, and the Pol•go• diidx: the larvae of both are trochospheres. The fourth class is Iliredinea, the leeches, which have the blood system communicating with the body cavity, and the external rings are four or live times as numerous as the inner segments. They have no seta and are provided with suckers. Consult Parker and Haswell, Zoology (New York, 1897). See E A WOU3I ; LEECH ; NEREIS : SERPULA ; WORMS; Fossu..