Adaptation
Adaptation is the fitness of an organism to its environment. It is the characteristic which results in suitable and convenient morphological and functional correlation between an organism and its environment.
Parasitic adaptations
The parasitic flatworms, the trematodes and cestodes, have undergone profound adaptations to suit their parasitic mode of life. These adaptations, termed parasitic adaptations in such cases, arc of morphological as well as physiological nature.
A. Morphological adaptations
1. Body covering, The thick tegument, frequently provided with scales, affords suitable protection to the parasite. It is probable that this thick protoplasmic layer is continually renewed by the cells
forming it.
2. Organs of adhesion. For a firm grip on or in the host’s body, some special organs of adhesion are necessary. The flatworm, for this purpose, are variously armed with suckers, hooks and spines.The suckers themselves may be with or without hooks and spines.
3. Organs of locomotion. Locomotion is actually an effort for procuring food. But parasites habitually inhabit such places in the host’s body, where suflicient food is available without effort. Thus, the
organs of locomotion, such as the cilia of free-living turbellarians, are absent in the parasitic forms. It is interesting to note that the locomotory organs are duly present in free-living larvae of parasitic forms; the miracidium possesses cilia and the cercaria possesses a tail for locomotion.
4. Organs of nutrition . Food of the parasite comprises the readily available digested and semi-digested food of the host. Elaborate organs for nutrition are thus not needed. Trematodes have an incomplete gut and, in most cases, a suctorial pharynx for sucking food. An eversible pharynx present in free-living turbellarians is absent in this case, asthe parasite has not to capture a large prey. In cestodes, the parasite freely bathes in the digested food of the host which is absorbed directly. There is thus total absence of alimentation in tapeworms.
5. Neuro-sensory system. Need for quick and efficient “response to stimuli” is associated with free active life and not with a quiet parasitic life in a safe environment. In parasites, therefore, there is profound reduction of nervous system and total absence of sense organs. Accordingly, the free-living miracidium possesses eye spots.
6. Reproductive system. It is the best developed system in helminth parasites, designed and perfected to meet the need for tremendous egg production. The parasitic flatworms, with a few exceptions like Schistosoma, are monoecious (hermaphrodite). Hermaphroditism is of distinct advantage to the parasite, because (i) it ensures copulation even when a few individuals are present (ii) after copulation
both the individuals lay eggs, thus doubling the rate of reproduction, and (iii) in the absence of a companion, the parasite can reproduce offspring. In cestodes, the reproductive system is much more elaborate; each mature proglottid possesses one as in taenia solium) or two (e.g., Cotugnia, Maniezia, Dipylidium) complete sets of male and female genitalia. In a gravid proglottid all other organs of the system degenerate to make room for the uterus which becomes highly enlarged and branched to accommodate a large number of eggs.
B. Physiological adaptations
1. Protective mechanism: The alimentary canal parasites have to protect themselves from the action of digestive juices of the host. The tapeworms accom plish this (a) by stimulating the walls of the gut to secrete mucus, which then forms a protective clothing around the parasite, (b) by secreting anti- enzymes to neutralize the digestive enzymes of the host, and (c) by probably continually renewing their protective body covering i.e., the tegument.
2. Anaerobic respiration: Environment in the gut and bile ducts is devoid of freqoxygen. The flat-worms inhabiting these places, therefore, respire anaerobically by breaking down glycogen.
3. Osmoregulation: The osmotic pressure of the endoparasite’s body fluids, especially in case of trematodes, is almost the same as that of the host. This renders osmoregulation unnecessary. But in the intestinal tapeworm the osmotic pressure is a little higher. This permits ready absorption of host’s digested food by the parasite.
4. High fertility: Eggs produced by a parasitic flatworm face a very uncertain future. While passing through the complex life cycle, these potential off springs face several hazards as a result of which a very small percentage of the total eggs produced reaches adulthood. This threat to the very existence of the species is suitably met by the parasite which in its life time may produce eggs in millions. The reproductive organs of the flatworms, as already noted, are accordingly developed. Additional multiplicative phases in the life-cycle of some flatworms further increase the output of protential offspring. Several cercariae develop from a single miracidium of liver fluke and a single hexacanth of Echinococcus produces several scolices, each of which is a potential tapeworm.
Adaptation is the fitness of an organism to its environment. It is the characteristic which results in suitable and convenient morphological and functional correlation between an organism and its environment.
Parasitic adaptations
The parasitic flatworms, the trematodes and cestodes, have undergone profound adaptations to suit their parasitic mode of life. These adaptations, termed parasitic adaptations in such cases, arc of morphological as well as physiological nature.
A. Morphological adaptations
1. Body covering, The thick tegument, frequently provided with scales, affords suitable protection to the parasite. It is probable that this thick protoplasmic layer is continually renewed by the cells
forming it.
2. Organs of adhesion. For a firm grip on or in the host’s body, some special organs of adhesion are necessary. The flatworm, for this purpose, are variously armed with suckers, hooks and spines.The suckers themselves may be with or without hooks and spines.
3. Organs of locomotion. Locomotion is actually an effort for procuring food. But parasites habitually inhabit such places in the host’s body, where suflicient food is available without effort. Thus, the
organs of locomotion, such as the cilia of free-living turbellarians, are absent in the parasitic forms. It is interesting to note that the locomotory organs are duly present in free-living larvae of parasitic forms; the miracidium possesses cilia and the cercaria possesses a tail for locomotion.
4. Organs of nutrition . Food of the parasite comprises the readily available digested and semi-digested food of the host. Elaborate organs for nutrition are thus not needed. Trematodes have an incomplete gut and, in most cases, a suctorial pharynx for sucking food. An eversible pharynx present in free-living turbellarians is absent in this case, asthe parasite has not to capture a large prey. In cestodes, the parasite freely bathes in the digested food of the host which is absorbed directly. There is thus total absence of alimentation in tapeworms.
5. Neuro-sensory system. Need for quick and efficient “response to stimuli” is associated with free active life and not with a quiet parasitic life in a safe environment. In parasites, therefore, there is profound reduction of nervous system and total absence of sense organs. Accordingly, the free-living miracidium possesses eye spots.
6. Reproductive system. It is the best developed system in helminth parasites, designed and perfected to meet the need for tremendous egg production. The parasitic flatworms, with a few exceptions like Schistosoma, are monoecious (hermaphrodite). Hermaphroditism is of distinct advantage to the parasite, because (i) it ensures copulation even when a few individuals are present (ii) after copulation
both the individuals lay eggs, thus doubling the rate of reproduction, and (iii) in the absence of a companion, the parasite can reproduce offspring. In cestodes, the reproductive system is much more elaborate; each mature proglottid possesses one as in taenia solium) or two (e.g., Cotugnia, Maniezia, Dipylidium) complete sets of male and female genitalia. In a gravid proglottid all other organs of the system degenerate to make room for the uterus which becomes highly enlarged and branched to accommodate a large number of eggs.
B. Physiological adaptations
1. Protective mechanism: The alimentary canal parasites have to protect themselves from the action of digestive juices of the host. The tapeworms accom plish this (a) by stimulating the walls of the gut to secrete mucus, which then forms a protective clothing around the parasite, (b) by secreting anti- enzymes to neutralize the digestive enzymes of the host, and (c) by probably continually renewing their protective body covering i.e., the tegument.
2. Anaerobic respiration: Environment in the gut and bile ducts is devoid of freqoxygen. The flat-worms inhabiting these places, therefore, respire anaerobically by breaking down glycogen.
3. Osmoregulation: The osmotic pressure of the endoparasite’s body fluids, especially in case of trematodes, is almost the same as that of the host. This renders osmoregulation unnecessary. But in the intestinal tapeworm the osmotic pressure is a little higher. This permits ready absorption of host’s digested food by the parasite.
4. High fertility: Eggs produced by a parasitic flatworm face a very uncertain future. While passing through the complex life cycle, these potential off springs face several hazards as a result of which a very small percentage of the total eggs produced reaches adulthood. This threat to the very existence of the species is suitably met by the parasite which in its life time may produce eggs in millions. The reproductive organs of the flatworms, as already noted, are accordingly developed. Additional multiplicative phases in the life-cycle of some flatworms further increase the output of protential offspring. Several cercariae develop from a single miracidium of liver fluke and a single hexacanth of Echinococcus produces several scolices, each of which is a potential tapeworm.
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