Definition of Parasitic Adaptation:
The parasitic adaptation can be defined as the profound changes and modifications occurring in per-suit of successful living so that the parasite is fully adapted inside the body of the host.
Cameron (1965) has stated that “the parasites continue to lead their life successfully by adopting various modifications and compromises—compromises in some respects parallel to those found among free-living sessile animals and those which have adopted monophagy.”
I. Morphological Adaptations:
Transformation from external to internal:
Parasitism undoubtedly began as a chance of contact of one organism with another. Sooner or later the guest began to partake the food procured by the host, becoming more and more dependent on such food and in many instances was gradually changed from an ecto-to endo-parasite.
Two important aspects of parasitism as encountered in the helminthes:
The structural and functional modifications in parasites depend on the degree of parasitism. In a successful parasitic group of animals the modifications run in two distinct directions—one leads to loss or degeneration while the other leads to gain or new attainment.
A. Degeneration:
The degeneration in helminthes particularly involves the locomotor, digestive and sensory organs. As the parasites live on the digested or semidigested food of the host, their organs of locomotion and alimentation have become simplified. They are mostly useless.
(1) Organ of locomotion:
Total reduction of locomotor organs is observed in adult except in the free-living larval phase when the ectoderm becomes ciliated, e.g., Miracidium and Hexacanth of flatworm.
(2) Organ of alimentation:
(a) Total disappearance in the adult tapeworm;
(b) In the hermaphroditic adult trematode it consists of a blind gut;
(c) In Redia stage it is further simplified and completely eliminated in the Sporocyst stage.
(3) Sensory organs:
The sensory organs are reduced or absent (e.g., Fasciola) in some endoparasitic platyhelminthes, and in some nematodes (adenophoreans) the sensory organs are poorly developed and represented by amphids. This condition can be correlated with the sedentary life of endoparasites in which they live, in a more or less uniform host’s environment.
B. New Attainment:
(1) Integument:
The integument covering the body of helminthes has become greatly modified to serve following three important functions:
(a) Absorption:
The phenomenon of absorption is striking in larval stages which develop in the lymph spaces of mollusca or in blood stream, muscle fibre or musculature of vertebrates (Cysticercus, Trichinella) and in the adult blood flukes in the hepatic portal system and in various species of liver flukes (e.g., Fasciola) in the bile tract.
In these cases, the body is leaf-shaped and dorsoventrally flattened, and the entire integument becomes thin and undoubtedly serves partly or fully as a means for food absorption.
(b) Protection against the digestive juice of the host:
In the case of the larval flukes which have to pass through stomach in order to reach the bile passage for further development—a cyst capsule is provided as a protection against the digestive juice.
Certain Amphistomes (in Ruminants) and Gnathostomes (in cats, dogs and horses) remain attached to the stomach wall. They are provided with thick resistant integument impregnated with chitin-like substances of impermeable nature.
(c) Protection against abrasion:
Many trematodes living in the intestinal tracts are provided with spinous integument to guard against the abrasive action of the food and roughage passing through the gut. These spines may be of accicular, dentate or placoid types and are rooted into the sub-integumental layer.
The oriental liver fluke Clonorchis sinensis, which was probably an intestinal parasite before it became a bile duct inhabitant, possesses a spinous integument during its larval phase— in fact, until it becomes safely located in the bile passage.
(2) Modification for attachment:
Essential prerequisite for parasitic life is the possession of suitable mechanism to attach strongly with host body.
Following modifications for attachment are often encountered:
(a) Acetabulum or sucking organ:
Found in all the adult flatworms which parasitise man:
(i) In the liver flukes (e.g., Fasciola) it consists of two suckers on the ventral side of the body—one anterior and the other posterior to it.
(ii) In the case of human tapewarm, it consists of either sucking tongue or groove, or four cups at the cephalic end of the worm.
(iii) In the tapeworms, the scolex bears four large suckers (Taenia solium) or accessory suckers (e.g., Myzophyllobothrium) or leaf-like outgrowth on the scolex, called bothridium. Phyllobothrium -has four bothridia, each bothridium with a sucker. Echinobothrium bears two bothria (It is a shallow groove on the scolex) and a spiny head stalk. Tetrarhynchus bears four bothria and four eversible proboscis bearing spines.
(iv) In some monogeneans, a highly specialized attachment organ at the posterior part of body called haptor (Opisthaptor) with suckers and hooks (e.g., Polystoma, Choricotyle, Polystomoidella) and an anterior adhesive organ (sometimes called prohaptor) consisting of suckers and adhesive glands (e.g., Gyrodactylus) are present.
(b) Hooks:
In some tapeworms and nematodes, hooks are situated in or around the anterior end:
(i) In Taenia, hooks are arranged in double circlet at the base of rostellum.
(ii) In the dog tapeworm (Diphylidium caninum), it occurs in several rows around the proboscis which may be everted.
(iii) Hooks are often provided with series of teeth and are placed in the buccal capsule.
(iv) In Macracanthorhyncus sp. a buccal armature of tooth-like structure is present, which serves for tissue aberration and anchorage.
(c) Glands:
In some of the helminthes there have been developed in the vicinity of mouth, unicellular secretory glands which serve in:
(i) Anchorage in favourable habitat, and
(ii) Aid in food supply.
(iii) In trematodes these unicellular glands, known as cystogenous gland, are more common in the Cercarial stage and serve the purpose of penetration to host tissue by elaborating histolytic substances.
(iv) In hookworms (Ancylostoma)— there are glands in buccal region which are supposed to have anti-coagulative and histolytic properties.
(3) Modification for reproduction:
The most conspicuous elaboration in organs and tissues in the helminthes is that of the reproductive system.
(a) Both Platy—and Nematodes have large part of their body mass occupied by these organs and their products.
(b) The adult flatworms, with few exceptions, are hermaphrodite.
(c) The roundworms are dioecious.
(d) Adult flukes and tapeworms have particularly complex reproductive organs. In both the groups, cross- fertilization, which was formerly the rule and is still a possibility, has been superseded by self-fertilization.
(e) In tapeworm, instead of a single body unit, there are multiple segments— proglottids, each one is sexually complete in itself.
(f) To ensure the perpetuation of the parasite species, endoparasites produce a large number of eggs.
The adult Ascaris lumbricoides produces 200,000 eggs daily. The human hook-worm Ancylostoma duodenale produces 25,000- 30,000 eggs per day. Each gravid segment of tapeworm contains 30,000- 50,000 eggs and the gravid segments may produce up to 1000.
Hymenolepis diminuta may produce up to 250,000 eggs per day throughout the life. Such an enormous amount of eggs which are produced by the endoparasites help to continue the race where the chances of survival are very remote.
C. The unaltered systems:
Two systems of organs—the nervous and excretory, have remained almost unchanged. However, the excretory system in the case of flatworms has undergone some insignificant changes.
Greatest modifications among the helminthes have been encountered in such forms that reside in the blood or lymph systems (blood flukes and filarial worms) or in the muscular tissue (Trichinella) or forms that attach to the peritoneum (Hydatid cyst).
They have been reasonably designated as old parasites while those forms which live in mouth or bladder of the host or on the body surface (Monogenic trematodes) have been termed young parasites as demonstrated by their relatively slight modifications from the prototypes of the group.
II. Physiological Adaptations:
1. Intracellular digestion:
Adult liver flukes, Fasciola hepatica feed on bile, blood, lymph and other nutrients of the host and digestion probably extracellular and takes place in the intestinal caeca. Reserve food is mostly in the form of glycogen and fat. They can take up glucose and other molecules through their body surfaces.
The species in which nutrients are absorbed through the body surfaces, is regarded as the intracellular digestion. Cestodes lack any form of digestive canal, so they feed on tissue elements and inflammatory exudates of the hosts. All nutrients are absorbed across the tegument. So digestion is intracellular.
In A. lumbricoides both extracellular and intracellular digestions have been reported. When the cells of the intestinal wall engulf solid particles for digestion, it is called intracellular digestion. Digestion is started extra- cellularly when takes place within the intestinal lumen but is completed intracellular.
2. Osmoregulation:
By the process of osmoregulation the endoparasitic helminthes maintain a relative constancy of balance of salts, ions and water in their tissues. Parasitic platyhelminthes such as cestodes and trematodes maintain the same osmotic pressure as that of their hosi, so there is no difficulty in maintaing life.
3. Anaerobic respiration:
The endoparasites live in an environment where there is more or less lack of O2. So they have become adapted in a low metabolic rate which requires a minimum amount of oxygen.
In this case the respiration is anaerobic type consisting of extracting oxygen from food which are absorbed in the tegument. In the absence of O2, energy releases by the fermentation of glycogen which is broken by glycolysis and form pyruvate or pyruvic acid (C3H4O3) as a hydrogen acceptor from NAD and forms lactic acid (C3H6O3) and CO2.
Conclusion:
Viewing the groups of parasitic helminthes as a whole with respect to successive stages of adaptation which they have undergone and are undergoing, one is able to appreciate the vastness and profoundness of the principles of adaptations and at the same time how marvellously the parasitism has become successful in helminthes.
