|Medicinal leech standing up under water|
The Medicinal leech is a species of leech that is best known for its medical purposes, hence its name. The substance found in the salivary glands of the medicinal leech is called Hirudin which interferes with the process of blood clotting. The leech is administered medicinally to promote the improvement of blood circulation in the certain organ. It also has anti-inflammatory properties, strengthens immunity, and raises nutrition of tissues. There are too many diseases to list that can be treated by Hirudotherapy, but of importance are diseases of heart, vessels, and chronic nonspecific diseases of lungs leading to liver and heart problems.
The medicinal leech has a flattened cylindrical body that is divided into 33 or 34 segments. The underside of this leech is spotted while the top is black or dark brown with six long reddish stripes. They are known for their disc shaped sucker at the end of their head which is used to draw blood. Once the leech has begun sucking, they would drink blood weighing as much as itself in fifteen minutes. Medicinal leeches range in sizes from 1-30cm in length, but most are between 2-5cm long. Gas exchange happens through the skin. The leeches have two brains, one at the tail end and the other at the head end. They have a looping movement and are very elegant swimmers. They flatten their body into a ribbon-like shape and thrust themselves forward in multiple vertical flutters. They are, for the most part, fluid feeders. They have strong teeth which make a Y-shaped cut in the skin. A full meal can take up to 200 days to digest. They are olive green with yellow, orange, red, and black lines on their back. Their jaws have many teeth that are skillful in piercing human skin. They have five pairs of eyes. Their cocoons are laid in damp soil. They are 30-35 mm but can stretch to 200 mm. Medicinal leeches are designed to aid in medical research. They suck blood and cure some blood clots.
They have a three part lifecycle: egg, larva, and adult. The larvae are not free swimming and remain in the albuminous fluid of the cocoon. These larvae look like eggs with mouths and don't have an anus. The organs of the larvae are lost and metamorphosis finishes development. Leeches are epimorphic. When the eggs hatch, the infantile leeches that arise from the cocoon have the same number of segments. Leeches exhibit two different growth patterns. They grow in spurts and the higher leeches display saltatory growth. These leeches go for periods of months between meals and have much slower digestion. The higher and lower leeches accomplish sexual maturity at different points in their adult lives. The lower leeches tend to deposit cocoons continually and begin to sexually reproduce when they reach a significant body weight. The higher leeches hit sexual maturity after a precise number of meals. Each leech has both male and female sex organs and can both give sperm to another leech and lay eggs. Each leech creates several cocoons holding eggs. A tough layer of protein protects the cocoon. They fasten their cocoon shrubbery underwater. Most develop faster in a warm climate. Some leeches can complete their lifecycle in a matter of months, but many can live for numerous years.
They necessitate warm-water ponds, with a variety of suitable hosts, especially amphibians, to live and thrive. Medicinal leeches flourish with a low population size. They can also be found in ditches with shallow margins. They like to be hidden so they would most likely be near dense, tall vegetation and moist conditions. The habitat must be suitable for host species, for example, sheep, horses, or cattle. The medicinal leech is haematophagus, a parasite feeding on only blood. The behavior of its feeding has been used for medical practices such as drawing blood from a patient. Between March and mid-October is when their prey searching behavior starts. Active food searching occurs only when the temperature hits around 45 degrees Fahrenheit. Their range extends over Europe and into Asia. They prefer muddy freshwater pools to live in. The over-exploitation by leech collectors in the 19th century caused them to be a scattered and reduced population. Drainage has also been a contributor to their reduction. Less water means less moisture which means poor living conditions. Another reason is that there has been a replacement of horses in farming, which are the leeches' main food supply. This species is now considered vulnerable. They are especially sparse in Belgium and France.
Behavior and Reproduction
Many leeches swim through the water with snakelike motions. They release their grip with the tail sucker and push off from a plant or rock, before moving their body back and forth. The leeches must mate to reproduce. Mating occurs when a leech attaches a sperm packet in the mate's body. Fertilization occurs in the female's body. As the cocoon passes over the female reproductive organs, the developing eggs are deposited inside. The cocoons are either attached to the bodies of other animals or left in the soil. Young leeches resemble the adult when they hatch. When the adult finds a host, like a frog or turtle, the young will hatch and also attach them to the host. 
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