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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.
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.
In invertebrates, the term parapodium (Gr. para, beyond or beside + podia, feet; pl.: parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids , where they are paired, unjointed lateral outgrowths that bear the chaetae .
The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes with cirri (fused bundles of cilia ) and gills .
The parapodia on segments 13, 14 and 15 are fused into three paddle-shaped, piston-like structures, the purpose of which is to pump water through the tube. The water is drawn in through the anterior end and expelled through the posterior end, [ 1 ] passing through the fine mesh of the mucus bag where food particles get trapped.
Each segment may bear setae (bristles) and parapodia (paddle-like appendages). Some species live freely, either swimming, crawling or burrowing, and these are known as "errant". Others live permanently in tubes, either calcareous or parchment-like, and these are known as "sedentary".
Setae in oligochaetes (a group including earthworms) are largely composed of chitin. [1] They are classified according to the limb to which they are attached; for instance, notosetae are attached to notopodia; neurosetae to neuropodia. [2] Diptera setae are bristles present throughout the body and function as mechanoreceptors.
The worms live out of sight in burrows under the sand and produce casts which are visible on the surface. Polychaetes, or marine bristle worms, have elongated bodies divided into many segments. Each segment may bear setae (bristles) and parapodia (paddle-like appendages). Some species live freely, either swimming, crawling or burrowing, and ...