Spionidae Larva belong to the phylum annelida, most marine annelids are in the class Polychaeta. Annelids are described as segmented worms and include species such as the earthworm. They are variable in appearance, but generally have chaeta (spines) or cilia.
Mostly annelids live on or in the sediment, however, some species have adaptations that allow them to live as plankton in the water column. These adaptations include small size, complex eyes and transparency. Spionidae are the most common family of annelids found in plankton samples, particularly in shallow habitats.