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Parapodia are mostly biramous (only the first two pairs are uniramous). Peristomium fused with the first body segment, with usually two pairs of tentacular cirri. The first body segment with 1-2 pairs tentacular cirri without aciculae. Compound setae are present.
A single stout internal chaeta, called an acicula, may be present in each lobe, which are used to support well-developed parapodia. Notopodia and neuropodia can also bear cirri which are tentacle-like projections of the parapodia. In some groups, such as the scale worms (e.g. Polynoidae), the dorsal cirrus is modified into a scale (or elytron). [2]
Nereis possess setae and parapodia for locomotion and gas exchange. [1] They may have two types of setae, which are found on the parapodia. Acicular setae provide support. Locomotor setae are for crawling, and are the bristles that are visible on the exterior of the Polychaeta.
In Polychaeta, chaetae are found as bundles on the parapodia, paired appendages on the side of the body. [1] The chaetae are epidermal, extracellular structures, and clearly visible in most polychaetes. They are probably the best-studied structures in these animals. [2] Segments bearing chaetae are called chaetigers. [3]
The micrometer-scale setae branch into nanometer-scale projections called spatulae. [6] A Tokay gecko's two front feet can sustain 20.1 N of force parallel to the surface using approximately 14,400 setae per mm 2. This equates to ~ 6.2 pN per seta, but does not sufficiently account for the overall stickiness behavior shown by the foot pads. [7]
Hence annelids' chetae are structurally different from the setae ("bristles") of arthropods, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle. [8] Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them.
The main body consists of repeating segments called chaetigers, each with a pair of parapodia containing setae, the function of which varies depending on where on the body the chaetiger is located. The body of D. claparedii may have between 35 and 150 chaetigers, and those near the posterior and anterior ends have hook-like setae to assist with ...
This large worm can reach 80 millimetres (3.1 in) millimetres in length and 10–12 millimetres (0.39–0.47 in) in width. It is buff in colour with purple specks. It lives in a tough, leathery tube covered with fine mud. Projecting from this is a branchial crown of branched tentacles, the radioles, which form a plume. The tentacles are striped ...