Classification of free-living and sedentary polychaetes relies almost exclusively on external characters, such as the shape of the head, and on the number and nature of structures, such as appendages (including anal ones), parapodia, and setae, and on tube construction. Oligochaete classification relies largely on internal structures, especially the arrangement and number of gonads, the position of the gonoducts, and particularly the location of the male pore. Setal characteristics are generally uniform among species. Leech classification is based on the presence or absence of setae and the nature of the mouth, proboscis (feeding organ), jaws, suckers, eyes, and reproductive system. ...(100 of 9920 words)