During the course of evolution, the endoparasites have acquired certain adaptations to endoparasitism for survival in the intestine of the predatory vertebrate host. These include changes in the structure and physiology as a result of continuous mutations. The prominent changes include:
Adaptation is the fitness of an organism to its environment. It is the characteristic which results in suitable & appropriate morphological & functional correlation between an organism & its environment.
The parasitic flatworms have undergone tremendous amount of modifications to adapt to their parasitic mode of life. These adaptations are known as the parasitic adaptations. Parasitic adaptations can be of two types namely morphological and physiological. The following is the description of both types of adaptations,
Body covering: The body of these animals is covered by thick covering called as tegument. This covering provided protection to the parasite. Also this tegument is continually renewed by the mesenchymal cells.
Organs of adhesion: These animals need a firm grip in the host body and for this reason special organs for adhesion are necessary in these animals. Accordingly these flatworms are equipped arms like suckers, spines and hooks.
Organs of locomotion: Generally animals need to move from place to place in search of food. But these helminth parasites locate themselves in such places inside the host body, where digested food is readily available. Thus the organs of locomotion like the cilia, flagella are absent in these forms. Some times locomotory organelles are present in the free-living larval forms. For example Miracidium larva has cilia and cercaria has a tail for locomotion.
Organs of nutrition: The organs of nutrition are also known as the trophic organs. The food of these helminth parasites comprises of digested or semi digested food readily available from the host. So special organs for nutrition are absent in these animals. Trematodes have an incomplete gut; an eversible pharynx is present in the free living larval forms. Cestodes on the other hand freely bathe in the digested food of the host so there is total absence of the alimentation in tapeworms.
Neurosensory system: These parasites have a reduced nervous system as there is no need for them to respond to stimuli quickly and efficiently. Also there is total absence of sense organs. Exceptionally for example, miracidium has eye spots.
Reproductive system: Reproductive system is the well-developed system among all the other body systems of helminths. The reproductive system is designed to perfected to meet the need for tremendous egg production. All these parasitic worms with an exception of Schistoma are monoecious or hermaphrodite. Hermaphroditism is an advantage to the parasite as it ensures copulation even when a few individuals are present and also after copulation, individuals lay eggs to double the rate of reproduction.
In Cestoda each proglottid consists of complete sets of male and female reproductive genetalia. Finally in the gravid proglottids all the other organs degenerate to make space for the uterus which becomes highly enlarged and branched to accommodate numerous eggs.
Protective mechanisms: These parasites which live in the alimentary canal need to protect themselves from the digestive effects of the digestive juices of the host. So for this reason, the tapeworms have special protective mechanisms like,
Anaerobic mode of respiration: As these parasites reside in the locations which do not have free oxygen, the respiration is anaerobic type by the breakdown of glycogen by the process of glycolysis. Glycolysis produces carbon dioxide and fatty acids.
Osmoregulation: Generally osmotic pressure of these endoparasites is equivalent to that of the host body fluids. Exceptionally the osmotic pressure of the intestinal worms is slightly higher to permit ready absorption of the digestive food from the host.
High fertility: The eggs produced by the parasitic flatworms have to face many challenges for the survival. While passing through the complex life cycle, these potential offsprings need to face severe hazards and consequently only a small number of eggs reach adulthood. This threat to the very existence and survival of these parasites is met by the mechanism of production of high number of eggs. And so the reproductive organs of these flatworms are developed to meet this need.
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