are worm-like parasites. The clinically relevant groups are separated according to their general external shape and the host organ they inhabit. There are both hermaphroditic and bisexual species. The definitive classification is based on the external and internal morphology of egg, larval, and adult stages. Helminth is a general term meaning worm. The helminths are invertebrates characterized by elongated, flat or round bodies. In flatworms or platyhelminths (platy from the Greek root meaning “flat”) include flukes and tapeworms. Roundworms are nematodes (nemato from the Greek root meaning “thread”).

1. Parasitic Adaptation In
Platyhelminthes and
Aschelminthes

2. Helminth
 The helminths are worm-like parasites. The clinically relevant groups are separated
according to their general external shape and the host organ they inhabit. There are both
hermaphroditic and bisexual species.
 The definitive classification is based on the external and internal morphology of egg,
larval, and adult stages.
 Helminth is a general term meaning worm. The helminths are invertebrates characterized
by elongated, flat or round bodies.
 In flatworms or platyhelminths (platy from the Greek root meaning “flat”) include flukes
and tapeworms.
 Roundworms are nematodes (nemato from the Greek root meaning “thread”).

4. Parasitism
 Parasitic worms, also known as helminths, are large macroparasites; adults can
generally be seen with the naked eye. Many are intestinal worms that are soil
transmitted and infect the gastrointestinal tract. Other parasitic worms such
as schistosomes reside in blood vessels.
 Some parasitic worms, including leeches and monogeneans, are ectoparasites -
thus, they are not classified as helminths, which are endoparasites.
 Parasitic worms live in and feed in living hosts. They receive nourishment and
protection while disrupting their hosts' ability to absorb nutrients. This can cause
weakness and disease in the host.

5. Parasitic adaptation
 The following points highlight the four main parasitic adaptations of helminths.
The adaptations are:
 1. Morphological Adaptations
 2. Physiological Adaptations
 3. Life Cycle Adaptations
 4. Immunological Adaptations.

6. Morphological Adaptation
 The Helminths, though are of lower grade of organisms, show
structural modifications or adaptations along two lines:
 (a) Degeneration or loss of organs or systems;
 (b) Attainment of new organs.

7. (a) Degeneration or loss of organs or
systems;
 1. Size
 Many parasites are large compared with their free-living relatives. This could be
related to increased egg production.
 2. Shape:
 Most parasites are dorso-ventrally flattened and this is related to the need to cling
on to the host. Fleas are laterally flattened and rely on escape through the hairs.
Nematodes are the obvious exception to the trend of flattening in parasites and
parasitic nematodes, as a whole, show little morphological specialization.
 3. Locomotor organs:
 As the adult parasites live for entire life in the body of the host, the locomotor-
organs are usually not necessary for them.

8.  4. Sense organs:
 In many parasites, particularly endoparasites,
there is often a reduction in the CNS and sense
organs.
 5. Alimentation:
 In endoparasites, again there is a trend to
reduce the gut and absorb nutrients through
the whole body surface.
 6 Glands in buccal region develop anti-
coagulatory secretion; e.g. Hook-worm.

9. (b) Attainment of new organs
 1. For entry inside the host:
 i. They secrete a liquid from unicellular gland to
dissolve host tissue and thus making a
microscopic passage for the parasite, e.g.
Miracidium larva of Fasciola sp.
 ii. Secretion also helps the hooklet to dissolve the
tissue; e.g. Hexacanth larva of Tapeworm.
 2. Organs for attachment:
 i. Hooks are arranged as a crown around the
rostellum in double rows, e.g. Taenia solium (Fig.
17.3).

10.  ii. Acetabula or suckers found in adult parasitic
flatworms like liver-fluke (Fasciola sp.) has two
suckers on the ventral side of the body—one
anteriorly and one posteriorly placed.
 iii. In Ophiotaenia—four suckers and sometimes
an apical one are also present.

11.  iv. In some Cestodes and Nematodes
hooks or hook-like structures develop
at the cephalic end, e.g. Coracidium
(Fig. 17.5).

 v. In Taenia solium the rostellum con-
tains a basal circlet of hooks.
 vi. In Dipylidium canium several rows
of hooks are present around the retrac-
tile rostellum (Fig. 17.6).

12. For protecton: cuticle
 i.Cyst membrane forms around the body, e.g., Metacercaria larva of liver-fluke.
 ii. The cuticle of Helminth is highly modified and adapted to resist against
digestive juices and for adhesion. The cuticle becomes thin, partly or wholely for
food absorption. The parasites live in rich nutritious environments; such as liver-
flukes (in bile), blood flukes (in blood), Sporocyst larva and Cysticercus (in
vertebrate muscles) and other larval forms (developing in lymph spaces and blood
stream).
 iii. In gut parasites, however as in Tapeworms, Gnathostomes, Amphistomes and
Nematodes—the cuticle becomes thick, impregnated with impermeable chitin like
substances and enzyme resistant, so that it is not digestable by the digestive juices
of the host but is permeable to water.
 iv. Presence of spinous integument—in many Trematodes.

13. 2. Physiological Adaptations:
 Physiologically Helminths show striking adaptation to lead the parasitic life in the body of the host and to
enjoy their life in simplest ways.
 They are:
 1. Intra-cellular digestion:
 The flukes feed on tissue elements and inflammatory exudates and have probably intracellular digestion.
 2. Osmoregulation:
 The osmotic pressure in the interior of the parasitic worms remains less than or same to the host, so that
there is no difficulty in exchange of water. Cestodes have well-developed water osmoregulatory systems
and their pH is also high.
 3. Anaerobic respiration:
 As intestinal parasites lead their life in an environment completely devoid of free oxygen (O2), evolu-
tionary adaptations have resulted in a very low metabolic rate requiring a minimum amount of oxygen.
Thus, respiration is anaerobic, which consists of extracting oxygen from food-stuffs—they absorb and
assimilate through their cuticle. But the manner of O2 liberation from food is not yet clearly known.
 In the absence of free oxygen, energy is obtained by the process of fermentation of glycogen, in which by
glycolysis it is broken down into CO2 and fatty acids. The glycogen and lipid contents in their tissues are,
therefore, high whereas the protein content is low.

14. Life Cycle Adaptation
 i. Simple in Turbellaria and Monogenic. Trematodes.
 ii. In digestive Trematodes a larval stage occurs.
 iii. In Cestodes one to three hosts may occur.
 iv. Nematodes may often show 1 to 2 hosts.

15. 2. Infection of secondary and tertiary
hosts:
This has three advantages:
 i. Increased reproductive potential, since asexual reproduction can take place in
the alternative host.
 ii. It increases the range of the parasite in space and time. That is infection of
more than one host which can increase the geographical range of a parasite,
particularly if one host is say terrestrial and the other aquatic. By infecting more
than one host species the parasite can survive periods when one host is tempo-
rarily scarce.
 iii. An intermediate host can channel the parasite towards its definitive host since
the intermediate host is frequently part of the final host’s food chain or else
closely related ecologically.

16. 5. Regulation of infection by the
host:
 Many parasites require a specific pattern of stimuli from their host before they are
able to infect them. This is particularly clear in those parasites that infect their
hosts passively via the gut in the form of cysts or eggs. Such stages may require
pre-digestion with host enzymes and the presence of specific bile salts as well as
the correct pH, temperature, redox potential, pO2 and pCO2 before they can hatch.
 6. Regulation of the adult parasite by the host:
 That is reproduction of the parasite is controlled by hormonal or physiological
changes in the host (e.g. the periodicity of microfilaria, and Polystoma in the frog).

17. 4. Immunological Adaptations:
 1. Absorption of host antigen
 2. Antigenic variation
 3. Occupation of immunologically privileged sites
 4. Disruption of the host’s immune response
 5. Molecular mimicry
 6. Loss or masking of surface antigens.
 Host parasite relation—the environment plays a key role.