From CreationWiki, the encyclopedia of creation science
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The Leech is a type of segmented worm best known for its blood-sucking parasitic behavior and stealthful ability to attach to skin undetected. Throughout the entire earth, there are over five hundred species of them. They can live in a marine environment, but they tend to be found in freshwater habitats.  There have been all sorts of stories, both true and fictional, about leeches throughout the centuries. Some of them are very convincing, and others have been quite exaggerated. When God originally created the earth, leeches were probably not parasitic, but after the Fall of Man, they probably evolved into a parasitic lifeform. It just goes to show you another way about how amazing God really is.
The leech is segmented into thirty-three sections, which are divided by a wall of connective tissue. A leech can be as short as 5 mm long, and can grow as big as 45 cm, or 18 inches. The two largest leeches in the world are each a foot long.  It has a dorsal-ventrally flattened body, as well as an anterior, and a posterior.  The posterior and the anterior usually have suckers, but sometimes it's on just the anterior. The suckers work through a combination of mucous and the suction generated by circular muscles.  One way for the leech to suck the blood of its host is to attach itself to the host with its anterior sucker and then insert a proboscis through their mouth and into the host's skin. Another way is to latch on with the mouth, which has three sets of jaws that make them very strong with a practically unbreakable grip. 
When the leech is sucking blood, it filters out the water in the blood inside the crop. That way it can store more of its food without becoming too big. It has special pockets for storing the blood, called diverticula, for future meals.  It has no parapodia, no setae, and is not divided up by septae. The body is completely filled with muscle and connective tissue.  There is no real kind of blood system for most leeches. 
The reproduction of the leech usually takes place between June and August. Leech mating occurs on land or in water. Leeches are hermaphroditic, which means that they possess both male and female reproductive organs. That means that they can mate with any other leech.  They use a mucus secretion to attach themselves ventrally, and the sperm gets injected to the vagina through an extendable copulatory organ. The egg sack forms near the clitellum, and then slips off the anterior end of the leech.  The cocoon-like eggsack is like a tough gelatin, and contains clitellum, along with other nutrients important to developing the baby leeches. 
The eggsack gets laid in the damp soil near the shoreline, and then the parent leeches either die, or live until the next mating season. After a second batch of eggs is laid, a leech will die. Leeches usually only live for one year, so if a leech were to mate again a year later, they would have to be a bigger and stronger breed of leech.  A few weeks after the the parent leeches leave, the baby leeches hatch as small, miniature leeches and go into the water to live their lives. 
The leech does not always have as great an effect on the environment as one might think. The majority of leeches are not the type that have permanent hosts, so most just have temporary hosts. There is one type of leech, the Horse Leech, that does not suck blood at all. Instead, it feeds on small worms, sucking them up through their mouths. The most dangerous kinds of leeches that you would ever find are in Southeast Asia, South America, Oceania, and Madagascar. They specialize in sucking blood from mammals. These leeches tend to be a pain for the people and animals of the area. The leeches are very fond of their blood, and they're usually waiting in a shallow area near the bank of a river, concealed by the muddy water. If you should happen to put your foot in that muddy water, it would come out with leeches on it.
There are also types of leeches that are quite misunderstood. The Dog Leech is one of the breeds of leech that does not feed on other organisms' blood. It merely sucks the water snail out of its shell with its sucker.  The ones that are parasitic though are dangerous. They feed on fish, reptiles, and mammals. To them, humans are a delicacy that they like to savor.
On the other hand, leeches are a good source of food for things like turtles, fish, ducks, and other kinds of birds. The leeches like to hang around the shallows because of predators. There they get hosts to suck blood from like human feet, deer hooves, and the feet of other things like raccoons, badgers, and foxes. That way they become the predator and not they prey. 
In history, leeches have been used for medical purposes. The doctors of the seventeenth century believed that if you drained some of the patient's blood, the infection would come out as well. They would put a few leeches on some part of the person's body, and then they would let them suck their blood. The leeches were also used to control swelling in a reattached limb of someone's body. People would make a living just hunting down leeches for medical purposes. They would find them, capture them, and sell them to physicians for money. 
The power of the leech to heal decapitated limbs is quite powerful. In 1985, surgeons used leeches to reattach a boy's ear that had been bitten off by a dog. It had been sown back on, but the small veins had become congested. So the doctor attached 24 leeches to his ear, and the leech saliva stuck the ear back together. 
- The Hirudinea Gordon John Larkman RAMEL, Earth-Life Web Productions, September 29, 2008.
- Subclass Hirudinea Phil Myers, The Regents of the University of Michigan , October 21, 2008.
- Leeches! Kate O'Laughlin, Water and Land Resources Website, May 3, 2007.