the most primitive bilaterally symmetrical invertebrates. All the scolecidan invertebrates have a primary axis of symmetry, an unsegmented body, either a primary body cavity or no body cavity at all, and excretory organs called protonephridia. The Scolecida are divided into three subphyla: Plathelminthes, Nemathelminthes, and Nemertini. These are often considered to be independent phyla.