Here's why:

* Oligochaeta is a class within the phylum Annelida (segmented worms).

* Earthworms are the most familiar and common members of this class.

* The name "Oligochaeta" refers to the fact that these worms have few (oligo-) bristles (chaetae) compared to other annelids like polychaetes.

While there are other members of the class Oligochaeta, earthworms are the most well-known and representative example.