蚂蟥，又称蛭、吸血虫，是环节动物门蛭纲的一类动物，雌雄同体。和其他寡毛纲相比，蚂蟥体外无毛，而且体腔的结缔组织更密集，因此身体更结实。

被发现的蚂蟥约700种，约100种生活在海洋中，约70种生活在陆地，余下都生活在淡水环境。蚂蟥是吸血动物，头尾各有一个吸盘，前段有口器，可刺入皮肤吸血，并有释放麻醉剂，因此不易被宿主察觉。吸血量非常大，是其体重的2—10倍。能耐饥饿。在野外草地中，蚂蟥有时会被误认为蛞蝓。

过去蚂蟥曾用于医疗，其唾液中有血管扩张剂，可有效防止血液凝固，减低因血液积聚所引起的高血压。在西方使用蚂蟥来医疗可以追溯到古希腊时期，而且一直沿用到19世纪。现在已经很少用蚂蟥做这种用途，但在整容手术或接驳手术中仍有使用。在德国被用来治疗骨关节炎。

Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like the oligochaetes, such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from the oligochaetes in significant ways. For example, leeches do not have bristles and the external segmentation of their bodies does not correspond with the internal segmentation of their organs. Their bodies are much more solid as the spaces in their coelom are dense with connective tissues. They also have two suckers, one at each end.