Leeches belong to the class of hirudines and are found in the sea, in fresh water and in humid land.
Like earthworms, they have a segmented body, clitellum and no differentiated head, much less parapodes. Are hermaphrodites and sexual reproduction includes the same steps described for earthworms. Unlike worms, however have no bristles on the segmentswhich is why they are also called annelids aquetas (= no bristles). The body is slightly flattened dorsiventrally.
The main difference, however, between leeches and other annelids is the presence of suckers fasteners that act as "sink plungers" and are located at both ends of the body. The anterior region houses the mouth and has some scraper denticles. The posterior end does not harbor the anus, which opens dorsally before the suction cup.
Most leeches, as the name makes clear, act as ectoparasite from other animals. Some species are predators of small invertebrates. As for locomotion, it occurs with the use of the two suction cups alternately, in a mechanism known as "palm-span", although many hirudines may swim through dorsiventral ripples of the body. In some lakes and streams in our country, it is very common to see vertebrate animals and even people coming out of the water with leeches, trapped in the buccal and nasal mucous membranes or on the skin.
A leech is capable of ingesting a blood weight three times its own weight. This way the animal can be without food for a long time and can often take up to 9 months to feed again.