Polychaetes are annelid worms that are mostly found in marine environments. Earth worms are annelids too, but they're oligochaetes. The oligochaetes are mostly found in freshwater and terrestrial systems.
The major differences between polychaetes and oligochaetes that can be used to distinguish them are the parapodia and chaetae. Polychaetes have them and oligochaetes don't. However, some aquatic oligochaetes have structures that look very similar to parapodia, and the parapodia and chaetae are hard to find on some polychaetes. There are exceptions to every rule!
|Cross section of a polychaete showing a typical body plan.|
Parapodia are fleshy outgrowths that are generally used for locomotion. In polychaetes the parapodia divide into upper and lower lobes. The upper lobe is called the notopod (pl. notopodia) and the lower lobe is called the neuropod (pl. neuropodia). Out of the lobes grow the chaetae, which are bristle-like in appearance. It's the many chaetae that give polychaetes their name.
Recently, the Telegraph newspaper published some electron micrograph images of some polychaetes that were collected from a hydrothermal vent. They make the already alien appearance of the worms even more strange. The false colour (EM images are black and white) does not help things.
|Portrait of a polychaete. I'm not sure, but I think it's a Nereid polychaete (rag worm).|
|Another polychaete. This one is a Polynoid polychaete (scale worm). I wonder if it's predatory?|
The reporting of these images has been pretty poor. Stories I have seen make it sound like these worms are only found in the deep-sea, which is not true. And that they eat the bacteria at hydrothermal vents, which is also not true. The images are of worms collected from deep hydrothermal vents, but there are many shallow water examples of both rag worms and scale worms. And both groups are generally predatory.
Rag worms and scale worms feed by rapidly everting their pharynx, which has some hardened mandibles attached. Prey that are caught in the jaws are dragged back into the mouth. In the upper image the rag worm still has its jaws hidden on the inside, while the scale worm in the lower image has partially exposed its jaws.
|The everted jaws of Perinereis cultrifera, a Nereid polychaete (photo estran 22)|
The other thing that was reported badly in some places involves the rag worm image. The structures that are coloured in a glowing pink are not eyes, they're sensory palps. They function as organs of smell, not sight. They eyes are on the upper surface of the animal and the image is of the underside.
|The head of a polychaete. The eyes are the four black spots at the left-hand end. The pharynx can be seen on the inside of the transparent body (photo Wim van Egmond).|