410 Worms, Annelida p lant mant 1st 1st ca gi p pr t gi pr/pe 1st t (a) (b) pr pe p 1st pr nep cc gi gi (c) (d) (e) Figure Polychaete anterior ends (a) Ampharetid, lateral view (b) Amphinomid, dorsal view (c) Arenicolid, lateral view (d) Cirratulid, dorsal view (e) Ctenodrilid, lateral view Modified from Rouse GW and Fauchald K (1997) Cladistics and polychaetes Zoologica Scripta 26(2): 139–204 of a few chaetae each Other polychaetes have chaetae arranged in flattened fascicles usually arranged dorsoventrally Each chaeta is produced from an invaginated epidermal cell (chaetoblast) The outer cell membrane of each chaetoblast is covered with microvilli, each of which produces a slender cylinder of chitin These cylinders are glued together by scleroprotein produced by cells lining the invagination All chaetae are formed the same way, through complex interactions between production of cylinders and glue secreted to keep the chaeta together and give the external shape The chaeta may be tapering capillaries, but most chaetae are much more complex (Figure 5) A species may have a single kind of chaetae or two or more kinds; in some parapodia each chaeta may be different, but more usually groups of chaetae with similar structure are present New chaetae are formed throughout the life of the worms, and distribution of different kinds of chaetae is carefully controlled and changes as the worm grows In rows of identical chaetae new chaetae are produced at one end of the fascicle and drop out at the other end (e.g., hooks in terebellids) In syllids that often have few chaetae, and each chaeta differs from all other chaetae in a parapodium, replacement takes place in a one-to-one fashion Special kinds of chaetae such as acicula, which are rods supporting long parapodia and various large spines often show signs of wear, an indication that they are not replaced or at least not replaced as often as other chaetae The base of the chaetae (chaetal sac) is anchored with muscles running across the body cavity, making it possible to protrude and retract the whole fascicle or bundle since in most cases only a single set of muscles are present Acicula and large spines often have their own investment of muscles and are independently movable The oligochaete taxa of the clitellates each have a limited number of kinds of chaetae, most have capillaries or spines The echiurans have minimally a pair of large hooks, usually located ventrally behind the mouth The sipunculans lack chaetae, but have epidermal hooks formed directly externally on the body