** Classification, Characteristics, Infection and Treatment


Helminths are a polyphyletic group composed of highly prevalent worms. Depending on the species, helminths may exist as parasites (that affect both human beings and animals) or as free-living organisms that live in nature.

Compared to such organisms as protozoa (that also exist as parasites and as free-living organisms), helminths are multicellular organisms that not only have differentiated/specialized systems, but also complex life cycles that go through several stages before reaching maturity.

The following are examples of helminths:


  • Giant roundworm
  • Threadworm
  • Pinworm
  • Roundworm Necator Americanus
  • Toxocara cati



Kingdom:  Animalia - Helminths are multicellular (eukaryotic) organisms and thus belong to kingdom Animalia. As such, they also belong to a group of animals known as metazoa.

While there is still confusion on how to group helminths in terms of taxonomy, they are divided into the following phylum:

Phylum Annelida


Annelida is a group/phylum that consists of segmented worms. Members of this group can be found in various different habitats across the world including deep marine, soil as well as in freshwater environments. 

Some of the organisms that belong to this phylum include:


Polychaeta (paraphyletic class of Annelida) - The least common members of Annelida. However, this group is composed of diverse organisms found in marine environments. 

Currently, there are about 8,000 identified species of class Polychaete that include:

  • Lungworms
  • Sea mice
  • Ragworms
  • Bloodworms


Members of class Polychaete are characterized by their well-defined segmented body that consists of a pair of parapodia that also consist of tiny bristles. 

Some of the other characteristics of Polychaetes include:

  • They range from a few millimeters to over 3 meters in length
  • They possess short sensory tentacles
  • Brightly colored
  • Mostly consume tiny aquatic plants
  • Some of the species are carnivorous
  • Divided into male and female (separate sexes). A few species have been shown to be hermaphroditic while a few others reproduce by dividing (budding)


Polychaetes are also divided into:


  • Free moving forms (errantia) - e.g. Polynoidae and Eunicida
  • Tube-dwelling Polychaetes (sedentaria) - They have reduced parapodia and highly developed filamentous anterior respiratory organs, examples of Sedentaria include terebellidae and pectinarridae



Oligochaeta - A subclass of the phylum Annelida (and class Clitellata) that consists of different types of aquatic and terrestrial worms. Today, well over 3,500 species of this subclass have been identified with earthworm being the most popular Oligochaete.

Oligochaetes can be found in such environments as marine environments (sea) fresh water as well as moist soil.

Some of the main characteristics of Oligochaetes include:


  • They range from a few millimeters to well over 3 meters in length
  • They lack a head as well as parapodia
  • They possess a few bristles
  • They are hermaphrodites
  • Compared to other helminths, they lack the larval stage and development and growth are direct


Hirudinea - A subclass of Annelida that consists of about 650 species.

Leeches are good examples of Hirudinea and have the following characteristics:


  • They possess a sucker (the sucker also contains the mouth part of the worm) at the anterior part of the body
  • They are segmented
  • The body length ranges from tiny to about 20 centimeters - However, some, like the Amazon leech can grow to 15 cm in length
  • They live in freshwater or can be found on land (moist soil)


In general, Annelida has the following characteristics:


Metamerism - They have three body regions (head, body, and pygidium) and a segmented body.

Chaetae - Thin-walled cylinders that consist of chitin.

Phylum Acanthocephala

Commonly known as spiny-headed (or thorny-headed) worms, Acanthocephalans are pseudoceolomic helminths that exclusively live in the small intestines of vertebrates (mammals, fish, and reptiles among others) as endoparasites (in their adult phase). 

This phylum is divided into three main classes that include:


Archiacanthocephala - The class Archiacanthocephala is composed of microscopic worms. A unique characteristic of Archiacanthocephala is that they have a body wall and a bundle of sensory fibers with nuclei that divide in the absence of spindle fiber. Like several other helminths, Archiacanthocephalas possess hooks which they use to attach themselves onto the gut walls of the host.


Eoacanthocephala- This is a class of Acanthocephala that is characterized by radial proboscis spines and ligament sacs among the female members. They are typically found in various aquatic cold-blooded animals (fish etc)


Palaeacanthocephala- Members of Palaeacanthocephala have been shown to live as parasites in birds, mammals and birds among other animals. They are characterized by possessing spines on the trunk with proboscis that are arranged in rows.


Whereas adult Acanthocephalans exclusively live in the gut of their hosts, the larvae have been shown to develop in the tissues of various insects and crustaceans.

While most of these organisms possess a proboscus that consists of retractable hooks, they lack a properly developed digestive system. For this reason, nutrients are obtained directly across the body wall through absorption.

With regards to reproduction, Acanthocephalans are primarily dioecious thus their sexual dimorphism. However, whereas the male Acanthocephalas possess a full complement of reproductive structures, adult females of these species only have a significant number of eggs.

* Members of this phylum rarely infect human beings.


Examples of Acanthocephala include:


  • Echinorhynchus
  • Plagiorhynchus
  • Moniliformis
  • Leptorhynchoides
  • Acanthocephalus
  • Macracanthorhynchus
  • Gigantorhynchus

Phylum Nematoda

Also referred to as Nemathelminthes, phylum Nematoda is composed of worms that are commonly referred to as roundworms.

Member of this phylum are distributed in a variety of environments across the world with a majority of them existing as free-living organisms. However, others live as parasites in plants as well as animals (including human beings).

A majority of nematodes can be found in marine and soil environments where they exist as free-living worms. However, others can be found in fresh water in lakes and streams as well as other extreme environments such as arctic ice and hot springs.

With regards to plant hosts, some of the nematodes can be found on such plants as herbs, shrubs and different types of trees where they obtain their nutrition.

* Some of the nematodes are beneficial due to their ability to insects.


Compared to some of the helminths (such as members of Annelida), nematodes have a simple non-segmented body that lacks any appendages. However, they have a complete digestive tract at some point in their development unlike members of Acanthocephala.

In addition, they also have a nervous system, excretory system but lack a circulatory system and a respiratory system.

Rather than circular muscles, nematodes have longitudinal muscles which allow for movement through contraction. This is enhanced by the fact that the internal pressure of the organisms is high which causes their body to flex and thus successfully thrash back and forth.

Some of the other basic body design of nematodes include:


  • Pseudocoelom body cavity
  • Non-cellular, multilayered cuticle on their exterior wall (flexible)
  • The alimentary canal is made up of a mouth, pharynx, anus, and intestine
  • Obtain and release gas through diffusion across their body surface
  • The majority are microscopic with the smallest measuring about 82um
  • The largest nematode can grow to reach 8 meters


Nematodes reproduce through a number of strategies, these include:


Amphimixis - Sperm and eggs from the male and female individuals to form a zygote.

Uniparent reproduction (and Autotoky) - A form of hermaphroditism where the sperm and the egg from the same individual fuse and develop. In some cases, the eggs have been shown to mature in the absence of any sperm.


* In some cases, the male produces eggs in the body following mating with the female. Young ones then break out through the body wall of the male resulting in its death.


Phylum Nematoda is divided into two main classes including:


Adenophorea - A majority of the species in this class are free-living organisms that can be found in marine and freshwater environments.


Secernentea - A majority of Secernentea are animal parasites.


Examples of nematodes include:


  • Ascaridida
  • Secernentea
  • Chromadorea
  • Enoplea


Some of the infections in human beings include:


  • Ascariasis
  • Hookworm enterobiasis
  • Filariasis
  • Angiostrongyliasis
  • Trichuriasis


Depending on the species and level of infection, some nematodes can cause the death of their hosts (human beings or animals) if not treated.


* While some nematodes can infect and affect some of their hosts, some are also beneficial in nature. For instance, some of the nematodes that live on land play an important role in decomposition while others act as predators of various microorganisms.

Phylum Platyhelminthes


The phylum Platyhelminthes is composed of flatworms that are more flattened in shape compared to other worms.

The phylum is divided into the following classes:


Catenulida - This is a small group that consists of about 100 species that have been identified so far. Most of the species belonging to this class can be found in such environments as streams, moist terrestrial habitats, and mires among others. Reproduction is primarily through budding (asexual reproduction).


Rhabditophora - Compared to Catenulida, Rhabditophora contains all the parasitic flatworms as well as a majority of the free-living organisms.


Platyhelminthes are also divided into the following parasitic divisions:


Trematodes - Also known as flukes, trematodes are shaped like a leaf and have an outer cover that may either be smooth (smooth all through the surface) or spiny. In their morphology, flukes also have two suckers (posterior and anterior).

Some of the other characteristics of trematodes include:


  • A majority of the trematodes are endoparasites that live within the host while a few species are ectoparasites
  • A majority are hermaphrodites that possess both reproductive organs
  • They have a well developed digestive system
  • For the most part, endoparasites survive by feeding on blood, tissue in their surrounding as well as mucus. However, this is largely dependent on the host
  • They need an intermediate host during their life cycle


Trematodes are further divided into three main groups that include:


  • Aspidogastrea - endoparasites
  • Monogenea - ectoparasites of fish
  • Digenea- endoparasites that may require one of two intermediate hosts


* Trematodes can cause serious diseases in human beings. A good example of this is Schistosomiasis, an acute/chronic disease caused by blood flukes. Infections can also result in organ damage which requires urgent care.



Cestodes - Like a good number of trematodes, cestodes are also internal parasites. They include such tapeworm species as Taenia solium and Taenia saginata.

In human beings, tapeworms attach to the intestinal tract by using a sucker positioned on the anterior end of the organism or using a scolex. The elongated body of the tapeworm (made up of proglottids) continues to grow as the tapeworm obtains more nutrients from its surroundings.

Tapeworm infection is typically associated with such symptoms as fatigue, nausea, general weakness (given that the tapeworm uses up most of the nutrients) as well as vitamin and mineral deficiencies.

They can also block the intestine as well as other smaller ducts of the intestine resulting in further serious complications. In some cases, tapeworm larvae have been shown to migrate to other body organs resulting in life-threatening infections.



Structural characteristics of Platyhelminthes:


  • Three embryonic layers of tissue (ectoderm, mesoderm, and endoderm)
  • Gastrovascular cavity in place of a properly developed digestive system
  • While some species have an anal opening, waste material is removed through the mouth structure of the flatworms
  • High branched gut sac
  • They have a nervous system that is made up of a pair of nerve cords running through the length of the body.

Helminth Infections

As already mentioned, helminth infections are caused by various species spread across different groups (phylum). These infections may be transmitted in a number of ways.

While some of the parasitic worms (larvae) such as Taenia solium infects human beings following ingestion of contaminated pork, others, like some species of nematodes, are soil-transmitted and often affect younger children. Infections may range from mild infections to serious, life-threatening infections.


* Good hygiene is an important preventive strategy that can help prevent most infections.


The following are some of the most common infections and treatment methods used:



This is a common infection in Western parts of Europe caused by pinworm and or threadworm. The worm may be transmitted from one person to another through contact (food items etc) or simply by consuming contaminated foods. It is rarely serious and often results in rectal itching. Mebendazole is typically used to treat this infection.


Intestinal obstruction and perforation (roundworm) - Ascaris can cause intestinal obstruction (ascariasis), perforation as well as volvulus. In some cases, the worm may spread to other organs where it can cause additional infections such as liver abscess and appendicitis. With such infections, treatment is with albendazole or given doses of mebendazole.



This is an infection caused by tapeworm and may be asymptomatic. However, advanced infection are associated with abdominal pain, nausea and weight loss among others. Infections may also spread to other parts of the body causing further complications. Treatment of this infection entails a specific dose (depending on the infection) of niclosamide and praziquantel.


Some of the other infections and their treatment include:

  • Cysticercosis (caused by pork tapeworm) - treated using Antiepileptic medication (e.g. carbamezapine, levetiracetum and phenytoin)


  • Schistosomiasis (caused by blood flukes such as S. japonicum and Schistosoma mansoni etc) - effective treatment involves using Praziquantel


  • Hookworm infections - treated using albendazole


  • Trichururiasis - caused by whipworm is treated by using Albendazole and mebendazole


  • Filariasis - a serious condition that results in swelling of different parts of the body (limb, genitals etc) caused by some nematodes. Early detection can help manage the infection.


Helminths are multicellular organisms most of which can be seen with the naked eye. However, helminth eggs, larvae and some of the species that are microscopic in nature can be viewed under the microscope.


Wet mount







  • Add a drop or two of saline at the center of the glass slide
  • Using a cotton swab obtain a small amount of the specimen and mix with the saline
  • You can add a drop of iodine for staining purposes
  • Gently cover the smear using a coverslip
  • Mount the slide on the microscope and view using 10× objective


Depending on the species, the larvae, eggs and microscopic helminth will take varying shapes (oval, elongated etc).

Learn more about Parasitology here

See a page on Flatworms, Nematodes

Of interest:  Parasites under the Microscope

Return to Multicellular Organisms

Return to Eukaryotes

Return from Helminths to MicroscopeMaster Home


DJ Richardson (2013). Acanthocephala. Quinnipiac University, Hamden, Connecticut, USA.

George Poinar (2006). Nematoda (Roundworms). 

Karin Kiontke and David H.A. Fitch. Nematodes. Current Biology Vol 23 No 19. 

Gilbert A. Castro. (1996). Chapter 86Helminths: Structure, Classification, Growth, and Development. Medical Microbiology. 4th edition.

Peter F. Weller. 255e: Introduction to Helminthic Infections. McGraw-Hill. 




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