Syllids are distinguished from other polychaetes by the presence of a muscular region of the anterior digestive tract known as the proventricle.

They are found in all regions of the ocean, from the intertidal zone to the deep sea, and are especially abundant in shallow water.

[3][5] They are found in a range of habitats, moving actively on rock and sandy substrates, hiding in crevices and among seaweeds, and climbing on sponges, corals, hydrozoans, seagrasses and mangroves.

[6] A young Syllid was one of the first worms to be found with pollen from seagrass in its stomach, making it a possible pollinator.

[4] The proventricle is composed of strirated muscle cells with the longest known sacromeres among animals.