In this article we will discuss about Parasitism:- 1. Meaning of Parasitism 2. Origin and Evolution of Parasitism 3. Kinds 4. Parasitic Adaptations.
Meaning of Parasitism:
Parasitism is defined as the association of two organisms of same or different species, in which one lives at the cost of other. A parasite is that which lives on other organism called a host, receiving nourishment and shelter without any compensation for the host.
According to Elton, the union of parasite and host is usually an elaborate compromise between extracting sufficient nourishment to maintain and propagate itself and not impairing too much the vitality or reducing the number of its host which is providing it with a home and free ride.
Origin and Evolution of Parasitism:
Parasitism dates back to ancient geological time and arose soon after the differentiation of life began in the world. Parasitism is a secondary mode of life. It has arisen from free living way of life. This get together of different individuals started with comparatively smaller animals occasionally taking shelter temporarily on the body of some larger forms.
When this temporary shelter was repeatedly used, the association of two organisms became more intimate and the smaller organisms got not only shelter, but also free transport and a bit of nourishment from the host, of course, without doing any harm to the host body.
Still later, the weak organism started living within the body of the host without doing any harm to the host. Finally they started to feed on the body tissues of the host so that the latter has to suffer with some harm. This association has been called parasitism.
It may be presumed that the parasite first confines itself to the external surface of the body and is called ectoparasite. The ectoparasites may gradually change to endoparasites when they get their entire nourishment from the host.
Kinds of Parasitism:
The parasitism can be classified into following categories:
Some free-living animals are capable of leading a parasitic life for a certain period in a host, if swallowed accidentally.
When the animals can live both as parasitic or free living, they are known as facultative parasites.
These include forms which have lost the power of living a completely free life, and must live on or in some other suitable organism during all or part of their lives, otherwise they die.
In ectoparasitism, the animals live on the external surface of the body of host and totally depend for food on the host.
In endoparasitism, the animals live inside the body of host and depend entirely for the food on the host.
The helminthes are modified morphologically as well as physiologically to live in their particular environment.
The Trematoda and Cestoda due to their parasitic mode of life show a departure in their anatomy and physiology from their free-living ancestor, the differences are pronounced especially in their locomotor, trophic and sensory organs; and more so in Cestoda than Trematoda.
1. The shape of the body becomes flattened like a leaf or a ribbon so that they can fit in the spaces where they have their habitat.
2. Cilia have disappeared entirely from the outer surface as they are no longer necessary.
3. There are no epidermal cells in the adult, instead of which the body is covered by a several-layered thick cuticle which protects the parasite from the juices of the host.
4. Organs of attachment such as suckers and hooks are formed by which the parasite is not dislodged from the host.
5. There are no organs of locomotion since they are not needed, the hosts transporting the parasites.
6. There is a reduction in trophic organs, and in cestodes, the mouth and alimentary canal have disappeared, since they absorb digested food from the gut of the host.
7. The nervous system is of a lower character than in the free-living forms and there is a complete absence of sense organs.
8. The reproductive organs are well developed and the production of eggs is prolific to ensure the continuance of the species; in cestodes the reproductive organs are repeated in each proglottis, and in some each proglottis has two sets of reproductive organs.
9. Some parasites have an additional multiplicative phase at some stage of the life cycle; in trematodes the rediae may produce daughter rediae, or the sporocyst may either divide by transverse fission or it may produce miracidium larvae; in cestodes there may be several generations of bladder worms as in a hydatid cyst.
10. Parasites find means of passing out fertilised eggs from the host into suitable places, the eggs have thick shells which are protective and prevent desiccation.
11. Most parasites have one or more intermediate hosts which act as transmitting agents to new final hosts.
A few physiological adaptations are also brought about in the body of the parasite, these are as follows:
1. The osmotic pressure of the body fluids of a parasite becomes the same as that of the host to prevent a disturbing exchange of water.
2. Parasites living in blood or tissues find an abundance of oxygen, but those found in alimentary canal or bile duct have a considerable tolerance to an absence of oxygen, they become modified to obtain oxygen by anaerobic respiration since in most internal habitats, the oxygen content is very low, they obtain their energy by an oxybiotic process in which glycogen is utilised as the source of energy, and the end products are carbon dioxide and fatty acids which are given off.
3. Cestodes stimulate the gut of the host to secrete mucus which forms a protective envelope around the tapeworms.
4. Gut parasites secrete anti-enzymes to neutralise the digestive juices of the host.