Haplosyllis spongicola is a small white segmented worm which lives parasitically on about forty known species of sponge around the world.

Each segment bears paired appendages known as parapodia with tufts of cilia known as cirri on the dorsal and ventral sides.

[2][3] Haplosyllis spongicola has a cosmopolitan distribution and is found in shallow coastal habitats in both temperate and tropical locations throughout the world.

[3] Haplosyllis spongicola lives on the inside walls of a number of species of sponges.

[4] Haplosyllis spongicola reproduces by means of headless stolons that grow out of the parapodia, each having a pair of black ocular spots and up to thirty segments.