There are some 17 phyla of marine worms.

Flatworms (Phylum Platyhelminthes)

Flatworm (3mm) on aquarium glass (60G)

Segmented Worms (Phylum Annelida)

Polychaetes (Class Polychaeta)

Polychaete literally means "many hairs" which is a good description of polychaetes. Most aquarists call them "bristle worms". There are 5,300 species and are very common in marine habitats. There are two basic forms of polychaetes, errant and sedentary. Errant polychaete are the typical "bristle worms" that are seen moving around the tank or burrowing through the sand bed. Sedentary polychaetes live in tubes which they build or in burrows in the sand.

Sedentary Polychaetes

Various feather duster worms (all 60G)

Various feather duster worms (all 50G1)

Tentacles of a terrebellid worm. The tentacles are sent out in search of particulate matter (detritus, etc.) which is captured and brought back to the worm.  These worms often expel a cloud of excrament. (50G1)

Errant Polychaetes

Polychaete (10mm) on glass. There are a number of these on the glass ranging from 5 to 20mm. These are translucent and you can see black pincer like jaws behind the head. (60G)

Polychaetes in sand (all 60G)

Polychaete in sand. This polychaete had burrowed right up to a burrow with two amphipods which then disappeared so I assume this polychaete is predatory. (60G)

Yet another polychaete in the sand (60G)

Polychaetes (60G)

Peanut Worms (Phylum Sipuncula)

Sipunculans are nonsegmented worms with a true coelom. A coelom is a body cavity between the body wall and gut that is lined with mesodermally-derived epithelium.

Sipunculan worms (60G)

Spoon Worms (Phylum Echiura)

The spoon worms are generally sausage shaped with a long proboscis. They are rarely seen in the aquarium as they normally stay in their burrows or hidden within the rockwork.

Spoon worm (50G1)