Taxonomists classify leeches in the phylum Annelida, which also includes earthworms. Roundworm is just another name for nematode, as it is used to describe all of the life forms within the phylum Nematoda. Hookworms and whipworms are other parasitic intestinal roundworms. Leeches are more advanced segmented worms that have a circulatory system, while roundworms do not. Scientists theorize that roundworms probably evolved into annelids about 545 million years ago.
Leeches use a proboscis to puncture the skin. The affected animal does not feel this because of an anesthetic released by the leech. Some leeches are predators that consume tissue as well as blood. The leech does not feed very often, so when it feeds, it consumes a lot. In medieval times leeches were used for "bleedings" which were the common "cure" for almost every type of ailment. These practices did not fare well with patients and were later discontinued due to scientific advances in medicine.
Parasitic roundworms are species specific in their living habits: a different species exists for dogs, cats and humans. Their eggs or larvae live in the soil and infect people through the mouth and skin, eventually settling in the intestines and causing abdominal pain, diarrhea and headaches. Other symptoms of roundworms infection in people are blood in the stool, pink eye and high fever. Weight loss and worms in the stool are also common symptoms of roundworm infection.
Aside from the creepy way that leeches and roundworms make people feel, these parasites are used in scientific research daily to develop new medicines. Leeches are used to help restore blood circulation in traumatized tissues to prevent the development and spread of gangrene. Anti-clotting serum is extracted from leeches and is being used to develop drugs for the treatment of heart disease.
Both roundworms and leeches are studied for their remarkable ability to regenerate their own nervous tissue! This phenomenon is called axonal fusion and refers to nerve axons that naturally repair themselves at the damaged sites, instead of regenerating the whole axon. At the time of publication, it is not known if this occurs in humans.