The leech class has about 400 species. Leeches are descended from small-bristled annelids. The body length of leeches is from a few millimeters to 15 cm.
Representatives of this class are characterized by the following features: a constant number of body segments (33), the presence of suckers, and the absence of bristles on the body.
Leeches mainly live in fresh waters, some in seas and moist soil. There are terrestrial species in the tropics. All leeches are either parasites that feed on the blood of vertebrates, or predators that eat small animals.
The body of the leech is flattened in the dorsal-abdominal direction, with two suckers – perioral and posterior. The mouth is in the center of the front sucker, while the back one serves only for attachment.
Outwardly, leeches are similar to other annelids, but they do not have setae. The method of movement of leeches is connected with this: they move, alternately attaching suckers to different objects and bending in a loop-like manner; many leeches are able to swim.
Parapodia, bristles, tentacles and gills are absent in leeches. On the anterior segments of animals, there are from one to five pairs of eyes.
Under the epithelium of the leeches, there are annular and very strong longitudinal muscles.
Blood-sucking leeches in the mouth have sharp chitinous teeth. The salivary glands of leeches secrete a special substance – hirudin, which prevents blood clotting both in the wound on the victim’s body and in the stomach of the leech itself (therefore, leech wounds bleed for a long time).
In the stomach there are so-called pockets for storing sucked blood. Therefore, the time intervals between meals in leeches can be very long – up to several weeks.
Breathing is carried out across the entire surface of the body.
Nervous system and senses
Leeches have a better developed nervous system than other worms. The sense organs perceive light, chemical, mechanical and other stimuli.
Leeches are hermaphrodites. After mating, they lay numerous cocoons with varying numbers of eggs.
Leeches serve as food for fish, can be hosts of parasites. Some blood-sucking leeches cause great harm to fish, birds, animals, humans.
In our weakly flowing and stagnant water bodies, the Great Pseudomonas leech is widespread. It feeds on the blood of various animals living in the water. For humans, this leech is not dangerous.
Another type – Medicinal leech – is used in medicine for hypertension and the threat of hemorrhage.