In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

The 90,000 species in this clade are traditionally divided into the paraphyletic class Adenophorea and the more natural class Secernentea. Free-living members can be found in moist soil and in decomposing organic matter on lake and ocean bottoms, where they play an important role in decomposition and nutrient cycling. This clade also includes parasitic members such as Ascaris lumbricoides, which infect over 1. 5 billion people, and Trichinella spiralis, the causative agent of trichinosis. For 10 points, identify this pseudocoelomate worm phylum whose most famous member is, the hermaphroditic model organism with exactly 959 somatic cells, Caenorhabditis elegans.
Science