Bristle  Worms    -    Class:   Polychaeta
      Bristle Worms are mostly marine segmented worms that have so-called parapodia (= fleshy protuberances) on each segment from which many chitinous chaetae (= bristles) protrude.   Ploychaeta are remarkable survivalists, they are found in the deepest trenches, near hydrothermal vents and in the coldest waters.  All of the roughly 10,000 described species - with more yet to be discovered - share the same basic body-plan but many do have various bodily adaptations to suit their particular lifestyle.  Most are bottom dwellers, either free-living or living in a tube, some are pelagic, some engage in commensalism and quite a few other are parasites.  Unlike Earthworms and their allies, the Bristle Worms do have a fairly distinct head also known as prostomium.
On the head various structures are located, these may include, two to four pairs of simple 'eyes',  a pair of palps, 'nuchal organs' and antennae.
      The earliest known Bristle Worm is Kootenayscolex barbarensis, a fossil of around 508 million years old found in 2012 in the Kootenay N.P. outcrop of the Burgess Shale in British Columbia, Canada. Well over 500 fossils of this species were found at the site which suggests it was possibly the most numerous annelid worm at the time.  The fact that it already looked very much like some of its' modern relatives suggests it had been around for a while.   There is also support for the notion that both Clitellata and Echiura (Spoonworms) are derived Polychaeta.   
Other fossils of Polychaetes of a similar age to K. barbarensis have been found in fossil-rich sedimentary rocks in Greenland.  The oldest fossil that appears to be affiliated with the Polychaeta is called Phragmochaeta canicularis estimated to be 520 My old.
Picture
Figure 1. Close-up photograph of the symbiotic vestimentiferan tubeworm Lamellibrachia luymesi from a cold seep at 550 m depth in the Gulf of Mexico. The tubes of the worms are stained with a blue chitin stain to determine their growth rates. Approximately 14 mo of growth is shown by the staining here. (Photo: Charles Fisher)
      Unlike Oligochaetes the Bristle Worms have separate sexes but most species reproduce using external egg fertilization. Fertilised eggs hatch into a trochophore (= a free-swimming planktonic larva) which possesses bands of cilia (= short hair-like protrusions) that are used both for feeding and locomotion through the water.  Quite a few species of Bristle Worms have rather different and more dramatic breeding habits that may include the adult literally exploding, releasing the gametes or adult individuals, en masse, splitting in two with one half floating to the surface where it disintegrates and eggs and sperm cells are released into the water. 
Bristle Worms are commonly no longer than 100 mm in length but there are a few extreme species: some no longer than 1 mm and at the other end there is the well-known 'Sand Striker" or 'Bobbit Worm', Eunice aphroditois, which can grow to a length of 3 meters and about 25 mm thick. It buries its' entire body in the sand and waits for prey to pass by which it catches with its' formidable jaws.
Picture
​Figure 2. 'An undescribed Polychaete from Marble Canyon.- Named in 2018 as Kootenayscolex barbarensis. From Parry et al, 2016. reproduced here under CC BY 04 Int.
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References  and  Links:
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​>     Figure  1.    Photo by Charles Fisher, reproduced in part only under CC BY 4.0 International.    Close-up photograph of the symbiotic vestimentiferan tubeworm Lamellibrachia luymesi from a cold seep at 550 m depth in the Gulf of Mexico. The tubes of the worms are stained with a blue chitin stain to determine their growth rates. Approximately 14 mo of growth is shown by the staining here. 
>      Figure  2.    'An undescribed Polychaete from Marble Canyon.'  Named in 2018 as Kootenayscolex barbarensis.  Image from Parry et al, 2016. reproduced here under CC BY 04 Int.

>      Boetius, A.,  (2005).   'Microfauna–Macrofauna Interaction in the Seafloor: Lessons from the Tubeworm.'   PLoS Biol 3(3): e102. https://doi.org/10.1371/journal.pbio.0030102
>      Naglu, K.,  and  Caron, J.-B.,   (2018).    'A New Burgess Shale Polychgaete and the Origin of the Annelid Head Revisited.'   Current Biology 28, pp 319-326, Elsevier Ltd.   https://doi.org/10.1016/j.cub.2017.12.019
>      Parry, L.A.,  Edgecombe,G.D.,  Eibye-Jacobsen, D.,  and  Vinther, J.,  (2016).    'The impact of fossil data on annelid phylogeny inferred from discrete morphological characters.'  Proc. R. Soc. B.283: 20161378 20161378    http://doi.org/10.1098/rspb.2016.1378

        https://www.sciencealert.com/kootenayscolex-barbarensis-cambrian-bristle-worm-burgess-shale
      https://whyy.org/segments/liz-bobbit-worm/​
      https://en.wikipedia.org/wiki/Burgess_Shale
        https://en.wikipedia.org/wiki/Chaeta
        https://en.wikipedia.org/wiki/Commensalism​
        https://en.wikipedia.org/wiki/Hydrothermal_vent
        https://en.wikipedia.org/wiki/Nuchal_organ​
        https://en.wikipedia.org/wiki/Parapodium
      https://en.wikipedia.org/wiki/Pedipalp​
        https://en.wikipedia.org/wiki/Pelagic_zone
      https://en.wikipedia.org/wiki/Phragmochaeta​
        https://en.wikipedia.org/wiki/Polychaete​
        https://en.wikipedia.org/wiki/Prostomium
      https://en.wikipedia.org/wiki/Trochophore​
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