Leeches (Hirudinea)

Leeches, the Hirudinea, are annelids, the segmented worms. Through most of the twentieth century Annelida was split into three major groups, the polychaetes, the Oligochaeta that include earthworms, and the Hirudinea. It is now recognized that Oligochaeta and Hirudinea, which together comprise several thousand species, form a clade, called Oligochaeta in some treatments and Clitellata in others. Earthworms and leeches are the familiar annelids for most people, while polychaetes make up the bulk of the diversity of the group and live in nearly every marine habitat, from intertidal algal mats downward.