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.
(3) The uncini are very short chaetae, which are simply embedded in the skin, and do not extend beyond the body-wall into the body-cavity. An uncinus is a sharply curved hook, which may have more or less numerous secondary teeth on it. They are characteristic of the Sabelliformia and the Terebelliformia.
The chaetae appear as solid, usually fibrillated structures, of a yellow or golden tint, transparent and refringent. Chemically they consist of chitin, and each chaeta is the product of a single cell. The chaetae of Euphrosyne are hollow and calcareous, being peculiar in both characters.
Fig. 138.—Chaetae of various Polychaetes (the magnification is not the same in all cases). A, Doubly-fringed capillary, from Terebellid; B, hooded crotchet, from Polydora; C, a fork, from Euphrosyne; D, jointed chaeta, from Phyllodoce; E, simple chaeta, with serrated ridges or frills, from a Polynoid; F, jointed chaeta, from Eunice; G, uncinus, from Pomatoceros (Serpulid); H, one of the outer series of paleae from the hood of Sabellaria spinulosa; I, jointed chaeta, from a Syllid; J, multidenticulate crotchet, from a Maldanid; K, comb-shaped chaeta, from Eunice; L, uncinus of a Sabellid; M, uncinus of Terebellid (Amphitrite Johnstoni); a, edgewise; b, side view; m, attachments of muscles into ba, basal plate; x, accessory teeth. N, Sheathed spear of Hermione; a, the spear-shaped capillary removed from its sheath; b, the same, with sheath.
Certain modifications of the chaetae presented by various worms deserve mention. In Polydora (Fig. 133, A) and in Chaetopterus (Fig. 173, p. [324]) those of one segment are especially strong, but their significance is uncertain. In Capitella those of the notopodium of the eighth and ninth segments are specially modified; they are analogous to the copulatory chaetae of Oligochaeta. In Aphrodite, in addition to the ordinary locomotor chaetae, there are brilliant, iridescent bristles and peculiar felting threads arising from the indistinct notopodium; these latter, however, are not true "chaetae," but are separate chitinous filaments similar to the constituent fibres of an ordinary chaeta.[[322]]
While the chaetae in the Nereidiformia and others are grouped in bundles, those of many other families are in vertical, transverse rows, as in Maldanidae and in Arenicola. The uncini are always embedded in such rows, usually slightly raised from the general level of the body surface, each being termed a "torus uncinigerus." These tori are usually limited to the sides of the body, but in Myxicola and in Notomastus they encroach upon the dorsal surface, and in Chaetozone, also upon the ventral, so as nearly to encircle the body, recalling the "perichaetous" condition of some earth-worms.