The helminths are modified morphologically as well as physiologically to alive in their particular environments. These modifications depend on the degree of parasitism.
(A) Morphological Adaptations:
The structural modifications or adaptations of helminthes have taken place along two lines.
(1) Degeneration or loss of organs.
(2) Attainment of new organs.
Endoparasites undergo simplification of unused organs or parts. In helminthes, the loss particularly involves the locomotory and digestive organs.
(a) Locomotory organs:
The helminth parasites live in the body of the host. So locomotory organs are quite unnecessary for them. So the locomotory organs are totally reduced except in larval forms.
(b) Alimentation: As the helminth parasites live on digested and semidigested food of the host, there is reduction in their elimination and digestive glands. The digested materials are absorbed directly in the body.
(c) Sensory organs: The sensory organs of helminths are also simple structures. Absence of complicated sensory structures can also be co-related to sedentary life in a sheltered habital, especially in the endoparasite.
(1) Attainment of New Organs: Parasitic existence leads to modification of old structures and attainment of new structures helpful in food absorption, protection, attachment and vast reproduction.
The outer integument or cuticle of helminth parasites becomes highly modified and is so adapted as to resist against the digestive juice, passage of food and for adhesion.
The well developed musculature in tape worms enables them to distribute their elongated snake-like bodies through-out the length of the intestine of their host. Similarly, power of locomotion enables the roundworms to counteract gut parasites and to maintain their position in the intestine. The advantage is that the worms cam obtains with greater ease of the pre-digested nutrients of the host.
(c) Organs for attachment:
Helminths are variously modified for adhesion to the body of their hosts. Suckers are formed in all parasitic flat worms’ foe adhesion. In some cestodes and nematodes, hooks or hook like structures also develop in or near the cephalic end which help in attachment.
In some helminthes, as in miracidium and cercaria, unicellular secretory glands develop which help during penetration in to the host.
(d) Vast reproduction:
The reproductive organs show significant development and adaptation to parasitism. There is vast increase in the reproductive capabilities through greater egg production.
Life history usually includes several larval stages for multiplication and for easy and sure transfer from one host to another.
The nervous system in parasitic helminthes and excretory system, particularly in trematodes, show little deviation or adaptation to parasitic mode of life.
(B) Physiological adaptations:
(1) Intracellular digestion:
The fluke as tissue elements and inflammatory exudates and have probably intracellular digestion.
The osmotic pressure of the parasitic worms remains less than or same as that of their hosts, so that there is no difficult in exchange of water. Cestodes have well developed water osmoregulatory system and their Ph tolerance is high.
(3) Anaerobic respiration:
The intestinal parasites live in an environment completely devoid of free oxygen. Their evolutionary adaptation has resulted in a very low metabolic rate which requires a minimum amount of oxygen. Moreover the respiration is of the anaerobic type, consisting of extracting oxygen from the food stuffs they absorb and assimilate through their cuticle. The manner in which oxygen is liberated from food is not clearly understood. In the absence of free oxygen, energy is obtained by the fermentation of glycogen,, which is broken by glycolysis, into carbon dioxide and fatty acids. The glycogen and lipid contents in their body tissues are high, where as the protein content is less?
(4) Anti enzymes:
Most of the helminth parasites, particularly intestinal parasites, secrete anti-enzymes in order to protect themselves from the gastric juices and digestive enzymes of hosts. In some strange evolutionary manner their ancesters developed these vital antienzymes and is quickly digested by the host. Some of the new medicines aim at nullifying the protective effect of these antienzymes resulting in the digestion of the worms by their hosts. In cestodes the bladder of the cysticerous larva is digested by the host’s digestive juices, but the scolex remains unaffected. The eversion of the scolex is also accelerated by the action of the host’s bile.
Successful adaptation to parasitism brings a sort of equilibrium between the host and parasite and consequently there is reduction in pathogenecity and where this equilibrium is disturbed, the parasite causes considerable clinical symptoms.