Another day, another worm. Today’s worm is the leech. Now I know that the word leech gives a lot of people the same creepy feeling as the words tick or head lice, but there is an upside to leeches. After all, a leech used to mean a doctor because the animals were such an important tool in the old doctor’s medicine cabinet. This was before modern medicine, of course, and, in an age of no antibiotics or anesthesia, medicine was pretty limited in what it could do for the patient. There were no microscopes either, so people didn’t know about the germ theory of disease. Sickness was often attributed instead to ‘bad blood’, hence the practice of bleeding people, either by just cutting them or by using the humble leech, Hirudo medicinalis.
Leeches were probably preferable because the risk of infection was less from the leech than the dirty knife. The leech injects a numbing agent and the leech can be removed before the patient bleeds to death, although leeches do inject an anticoagulant into the wound, so bleeding can go on after the animal is removed. Leeches went out of fashion for a number of years after the germ theory took hold, but lately one sees articles in the press and on the internet about medicinal uses of leeches. For instance, a leech is still a good way to remove the congealed blood in a bad bruise.
But let’s get back to basics. Leeches are segmented worms belonging, with earthworms (Oligochaetes) and marine segmented worms (Polychaetes), to the Phylum Annelida. The reason leeches get their own group, the Hirudinea, is because they are so highly adapted to a blood-sucking lifestyle that they no longer fit conveniently into either of the other groups, although their affinities to the Oligochaetes are still apparent.
All annelids are bilaterally symmetrical with segmented bodies and body parts (metamerism). The eggs undergo spiral indeterminate cleavage and the larval form is a ciliated free-swimming trochophore type. The adult is worm-shaped and segmented with a body wall covered in epidermis and inside this both circular and longitudinal muscles which move the segments. Each segment may have setae or hairs which assist with moving the body. Inside, there is a proper coelom, a closed circulatory system, a pair of nephridia for excretion, a complete one way gut, and a head end with a mouth.
The leeches have then added their own specialties for the chosen lifestyle of many species, that of blood sucking ectoparasitism. Their most notable characteristic is a pair of powerful sucker discs, one at the head end, with the mouth in the middle, and one at the posterior end, containing the anus. All leeches have 34 segments which is interesting because segment number is quite variable in both the Oligochaetes and the Polychaetes.
About 500 species of leeches have been described worldwide and most of these are freshwater organisms. Surprisingly, a number of these species do not suck blood at any stage of their lives, but survive by predation instead. Hosts of freshwater bloodsucking leeches include fish and frogs, crustaceans and snails, as well as mammals and birds. There are a few marine leeches, including a genus that attach themselves to shark gills, and a few terrestrial species that survive in high moisture environments such as mosses.
Most leeches respire directly through the body surfaces but a few have gills. Most circulate the gases plus foods through coelomic fluids although the coelom itself is reduced in size. Leeches have good sense organs including eyes which may be simple photoreceptors or more complicated structures. They can also sense water currents, touch, and, possibly, the heat of their prey in blood-sucking species that attack birds and mammals. They process this information with a nerve cord and an anterior nerve ganglion.
Leeches use muscles and suckers for movement and have done away with the normal annelid bristles. Usually they attach by the back sucker and wave the front end around until they sense in what direction they want to go. When they make up their tiny minds, they attach the front sucker in that direction and use their muscles to pull the body up, then throw the back sucker over the front and attach it again in a curious rolling gait similar to that of an inchworm or a slinky. Some aquatic leeches can also swim by undulating through the water.
On finding a suitable host, the mouth in the oral sucker makes an incision, an anticoagulant named, appropriately, hirudin, is injected into the wound and the leech can begin to feed. The muscular pharynx pumps the blood into the stomach. Leeches can swell up with blood to five times their normal size before dropping off, preferably back into the water, where they rest and digest their meal. Predatory leeches attack animals like earthworms and often swallow them whole.
Leeches reproduce sexually with separate males and females. Females have a single pair of ovaries and a gonopore for releasing the eggs. Males have from four to ten pairs of testes leading through sperm ducts to the gonopore. Leeches produce spermatophores that are driven into the female’s body and then discharge the sperm which swim to the ovaries for internal fertilisation. Leeches secrete cocoons to hold the young and may brood these. The eggs develop into ciliated larvae and then into adults, although some larvae already have a posterior sucker when they emerge and may attach themselves to their mother for aeration and protection.
Leeches are most likely to be encountered when swimming in farm dams or walking through wet habitats such as swamps or rainforests. They can be secretive and camouflaged and, by numbing the host when they attach and make the incision, go undiscovered for some time. The best way to remove them is just to pull them off rather than putting nasty chemicals or salt on them, which can make them vomit into the wound. I always release the leeches after removal as they are a natural part of the ecosystem and probably have as much right to be there as we do.
“References: http://www.austmus.gov.au/factSheets/leeches.htm”:References: http://www.austmus.gov.au/factSheets/leeches.htm Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.