Of the typical dorsal and ventral cirri, the ventral is only absent in some Amphinomids amongst the Nereidiformia; the dorsal is absent in _Nephthys_ and degenerate in _Glycera_, whilst in a very large number of families of the other sub-Orders neither cirrus is present. These cirri, though originally filamentous and sensory, may, by virtue of special blood supply, become "gills," and this occurs in several families of different sub-Orders. Thus in _Eunice_ this gill is comb-like; in _Amphinome_ and in _Arenicola_ (on certain segments) it is arborescent, as it is also in one to three segments in Terebellids; whilst in Ariciidae, Spioniformia, Cirratulidae, Opheliidae, and _Sabellaria_ it remains more or less finger-shaped or filamentous. In the family Serpulidae the thoracic cirri, both dorsal and ventral, {266}become flattened and extended antero-posteriorly, and unite with one another to form the "thoracic membrane."[320] In Phyllodocidae the cirri are foliaceous and natatory, and they contain a great quantity of glands of a peculiar character. The Aphroditidae are distinguished from other Annelids by the possession of "elytra" or dorsal scales, which appear to be the dorso-ventrally flattened cirri, retaining their sensory nature, but adding to this function several others.[321] The CHAETAE or bristles are mainly used in locomotion, but it is not unreasonable to believe that some of the stronger, serrated kinds may be used as weapons of offence and defence; certainly the Polynoids, bristling as they do with stiff chaetae along each side, must be rather unpleasant to their smaller enemies. The various bristles may be placed in three chief groups, viz. (1) simple; (2) jointed; (3) uncini (see Fig. 138). (1) The simple chaetae may be smooth and hair-shaped, _i.e._ "capillary," such as are present in nearly all families: or they may be forked (Amphinomidae), comb-shaped (_Eunice_), notched or serrated, or provided with a series of frills at right angles to their length, as in Aphroditidae; or fringed along one or both sides with a membranous expansion, as in Terebellids and Sabellids. The simple chaetae may also be short and spine-like, as in the ventral bundles of _Arenicola_; or they may be slightly curved at the end and notched, forming what are generally termed "crotchets," such as are common amongst Oligochaeta. These "crotchets" may be simple, or have numerous denticulations at the end (Maldanidae), or be provided with a membranous hood (Spioniformia, Capitelliformia). In _Hermione_ peculiar sheathed, spear-like bristles occur (Fig. 138, N). (2) Jointed chaetae have already been described (p. 246); they are confined to the sub-Order Nereidiformia, and occur only in certain families.