Nematodes.

Class: Nematoda

Phylum Adenophorea

OR

Secernentea

Nematodes are divided into two classes; Andenophorea and Secernentea and their phylum is Nematoda.

Some classes in the phylum Nematoda include Enoplea and Chromadorea. These classes are based on morphological and genetic differences among nematodes.

It seems like you are referring to the taxonomic classification of an organism. A rounded worm can belong to the Kingdom Animalia, Phylum Nematoda, Class Secernentea, Order Rhabditida, Family Rhabditidae, and possibly the genus Caenorhabditis if you are specifically referring to the model organism C. elegans.