This is a symbiotic relationship between two organisms in which one species (parasite) benefits for growth and reproduction to the harm of the other species (host). It must be emphasized that parasite and host interact and that excessive harm done to a host, which makes it less competitive , also endangers the survival of the parasite species. Parasitism can be differentiated into
depending respectively, on whether they live on or in the host. Lice, flea, ticks, etc. are examples of ectoparasites. Tape-worms, bilharzia and the malaria parasite are examples of endoparasites.
A plant parasite is the Dodder (Cuscuta sp.). It is often found twined round the stems of clover plants or grasses, which it damages severly. Each Dodder plant consists of a long slender yellow or pinkish stem, with the leaves reduced to tiny scales, and roots being absent except for a short time after germination. At intervals along the twining stem small rootlike structures, called haustoria, link the Dodder to its host and penetrate to the host's vascular bundles. In this way the parasite obtains organic nutrients, water and mineral salts directly from the host. The Dodder is not fussy about its host. Any herbaceous plant can be infected. Woody plants are usually too hard for the haustoria to be able to penetrate. If plants are densely packed, dodder will spread rapidly to adjacent plants. It can cause a great deal of damage to wheat or lucerne fields.
The bilharzia parasite, Schistosoma haematobium, a parasitc flatworm, is a good example of a successful parasite. It completes its life cycle in two hosts. The male and female adults live in the blood of humans while larval forms live in the bodies of a type of snail, Bulinus africanus. The adults posses suckers with which they attach themselves to the walls of blood vessels. Their bodies are covered with thick cuticles. When mature, adults meet in the blood of man. The male and female become 'associated' in that the slightely broader male rolls its body into a tube in which the long, thin female lives. When the female is ready to lay eggs she frees herself and moves into small blood vesels in the wall of the bladder. There she lay eggs. When the egg comes into contact with the water, its shell breaks and a ciliated larva, called a miracidium, is released. If it comes in contact with a host it works itself into the body of the snail by means of hydrolysis. Sporocysts are produced by the miracidium. Cercariae are produced after several generations of sporocysts. The cercariae make their way into the water and make contact with a human. Their it comes into the blood stream and live their. Within six to twelve weeks the larvae develop into adults and the cycle is reported once more.