At first glance, leeches and slugs are similarly nasty elongated, slimy creatures. Despite their similar body forms, they are different from each other. Leeches are related to earthworms; most of them feed on the blood of other creatures. They have been used in clinical bloodletting for centuries. Slugs are mollusks -- they're basically snails without shells. They feed on plant material, and some are serious pests of food crops.
Anatomy – Leeches Are Segmented and Slugs Have a Foot
Leeches belong to the phylum Annelida; they are segmented worms. The surface of the leech has 102 circular rings; suckers are at both ends of the body. They move in a crawling motion using the sucker at each end. Slugs are gastropod mollusks with two pairs of feelers on their heads. One pair has eyespots, and the other has a sense of smell. Slugs have a muscular foot on the underside and move by muscular contractions of this area.
Habitat – Both Like Moist Areas
Most leeches live in freshwater, but some species are terrestrial or marine. Terrestrial leeches live in low foliage in wet rain forests or in other moist places. In dry weather they burrow into soil and reactivate when they feel rain. Slugs mainly live on land, although there are some sea slug species. They like damp areas in gardens, fields and woodlands that are protected from the sun, and they are most active at night.
Food – Blood vs. Plants
The majority of leeches are bloodsuckers who prey on vertebrates and invertebrates. They are opportunistic feeders and will feed on any creature if their preferred host is not available. Most secrete an enzyme that prevents blood from clotting and acts as an anesthetic so the host does not feel them. They can consume more than their own weight of blood at one time, then drop off the host to digest it. Slugs feed on the foliage of a wide variety of plants including fruit and vegetables. Some will eat carrion and earthworms.
Reproduction – Both Are Hermaphrodites
Both leeches and slugs are hermaphrodites, which meas each specimen has male and female sex organs. After mating, fertilized eggs are deposited into a tough rubbery cocoon that is attached to a rock or log, or sometimes buried. The young emerge after several weeks. Slugs begin life as males and develop female organs as they mature. They lay eggs in holes in the ground or under wood or rocks. They generally hatch within 14 to 30 days.
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