Phylum Platyhelminthes consists of over 20,000 species found in terrestrial and aquatic environments.
The majority of species are parasitic and thus found in association with other organisms. These include domestic animals, mammals, and fish (particularly freshwater fish).
Also known as flatworms, they are characterized by a dorsoventrally flattened body with bilateral symmetry. While they share several key characteristics, they significantly vary in size and shape and exhibit a variety of colors.
Like many other animals, phylum Platyhelminthes belong to the kingdom Animalia and division Eumetazoa. They are also placed in the clades Bilateria and Protostomia and are divided into four main classes namely, Monogenea, Cestoda, Trematoda, and Turbellaria.
* Bilateral symmetry - Means that if divided into two (lengthwise) the opposite sides will be similar to each other.
Some examples of phylum Platyhelminthes species include:
As mentioned, phylum Platyhelminthes are divided into four main classes. Though they share several key characteristics, they also exhibit various differences that will be discussed below.
Currently, there are an estimated 5,000 to 6,000 Platyhelminthes species in the class Monogenea. Members of this group are characterized by the presence of an attachment structure known as a haptor.
Located at the posterior part of the body, the haptor consists of a hook that allows the organism to attach to the host. This feature differentiates monogeneans from turbellarians, trematodes, and tapeworms. Aside from the haptor, some of the species also bear a sucker (weakly developed in some species) that aids in attachment.
Monogeneans are commonly found in aquatic environments where the majority of species exist as parasites of fish and various invertebrates.
Despite having haptors and suckers, there are many species that live as browsers. As such, they feed on the mucus and epithelial cells of fish and various invertebrates without being permanently attached to the host. Others, however, remain permanently attached to the host (on the skin or gills of fish) and continue feeding off a specific site.
As they are generally not present in high numbers, they do not normally kill or cause disease to the host. Disease and high mortality occur when they increase in number as shown in experimental studies.
* The majority of monogeneans are ectoparasites. As such, they do not invade the body of the host (they might be attached to the external surface or the host or only feed on it).
* In high numbers, they are responsible for small ulcerations, discoloration, and inflammation and can cause the death of the host.
* A few species may invade the body and enter various parts of the body including the ureter, vascular system, and various body cavities.
Monogeneans can be found in both fresh water and marine environments where most of their hosts reside.
The adult forms are hermaphroditic thus each individual has both male and female reproductive structures (self-reproduction is very rare).
Following fertilization, the embryo gives rise to young ones with the ability to attach to the host. This is especially common among gyrodactylids.
As for oviparous monogeneans, the eggs possess appendages that slow the movement of water in the column before ciliated larvae are released. These larvae then attach to the hosts in their surroundings. Among some species, the eggs are sticky and can easily attach to substrates or parts of the host like the gills.
Some members of the Class oviparous monogenea include:
* Monogeneans have a simple lifestyle that only requires one host. The free-swimming larvae attach to the host which they depend on for nutrition as they develop into adult forms.
The class Cestoda (cestodes or tapeworms) consists of over 5,000 species. Unlike monogeneans, cestodes can be found wherever vertebrates exist. For this reason, they can be found in aquatic and terrestrial environments.
Morphologically, their bodies are divided into three main sections which include the scolex/head (consists of the rostellum, hooks, and suckers), the neck (which joins the head to the body), and the proglottid.
The neck of cestodes is an important part in that it is the part from which new proglottids are formed as the organism grows in length. The segmented part of tapeworms is referred to as strobila. The oldest proglottids are detached from the strobila as new ones are formed.
A single individual can grow to lengths of between 4 and 10 feet in length. However, some can grow to be over 40 feet long.
Cestodes are endoparasites and thus live inside the body of the host (they commonly reside in the gut). Here, suckers and hooks allow them to remain attached to the walls. They have a protective cuticle (a tough cuticle also known as tegument) that also aids in the absorption of digested food (they do not have a digestive system).
Each of the proglottids consists of male reproductive organs (testis and vas deferens) as well as the female reproductive organs (ovary uterus, and yolk gland). Also present in each of these segments is a genital pore for each of the reproductive systems. Though they are capable of self-reproduction, proglottids of individual tapeworms can fertilize each other.
Their life cycle involves three main stages, eggs, larvae, and adult forms. The adult forms are commonly found in the gut of the definitive host (e.g. human beings) while the eggs and larvae are commonly found in the intermediate host. Eggs hatch in the intermediate host to produce larvae that undergo encystation in various organs or the musculature. The larvae transfer once they are ingested by the definitive host (e.g. in poorly cooked meat).
* In human beings, the Cestode T. solium can cause cysticercosis while Echinococcus multilocularis can cause alveolar disease.
Some examples of Cestodes include:
The class Turbellaria consists of about 5,000 species found in aquatic environments. The majority of species can be found in marine habitats but some can be found in freshwater habitats as well as terrestrial habitats that are moist (e.g. under rocks in springs and streams).
Anatomically, they are smaller in size as compared to cestodes; averaging about 10 mm in length (a leaf-shaped body shape).
The body of a planarian (representative member of the group) consists of a head region with two eyes and auricles (chemoreceptors) and a ciliated body covered by an epidermal layer. The mouth part is located on the ventral side of the body more to the posterior end than the anterior at the mid-ventral line.
They are capable of swimming in water as well as creeping on various surfaces using ciliary propulsion.
Unlike most of the other Platyhelminthes, turbellarians are free-living organisms that feed on smaller invertebrates such as tiny snails, worms, and protozoa. Digestion occurs in the gastrovascular cavity within the cells that line this cavity. Undigested material is eliminated through the mouth because they do not have an anal opening.
Reproduction can occur sexually through copulation or asexually through transverse fission. Though turbellarians like planarians are hermaphroditic, sexual reproduction usually occurs through cross-fertilization.
During copulation, the penile organ is inserted into the genital pore of the other individual for fertilization. The eggs are then contained in a cocoon before they hatch to produce juvenile planarians. Asexual reproduction, on the other hand, involves the splitting of the individual. The individual halves then regenerate the missing half and the cycle repeats.
* Planarians have a nervous system that consists of two longitudinal nerves that run along each side of the body. These nerves form a simple brain in the anterior portion of the body.
* They also possess flame cells (bulb-like structures) involved in excretion.
Some examples of Turbellarians include:
The Class Trematoda (trematodes) comprises over 20,000 species.
Also known as flukes, trematodes are internal parasites of various animals including mammals (particularly cats and dogs), birds, and mollusks. Outside the body, they can be found in freshwater habitats inhabited by snails (their intermediate hosts) and untreated waste.
Morphologically, flukes are characterized by a leaf-like shape and are generally small in size, ranging from a few millimeters long to about 8 centimeters in length. F. buski, one of the largest flukes can grow up to 75mm in length and about 2cm in width.
In most species, e.g. Fasciola hepatica, the anterior part of the body is generally broader compared to the posterior end. They are also characterized by an oral sucker, located at the anterior portion of the body), as well as a ventral sucker often referred to as acetabulum.
Like the other Platyhelminthes, trematodes are dorsoventrally flattened. Their bodies are covered with a tegument which might contain scale-like spines in some species. The mouth part, located in the anterior portion of the body, connects with the esophagus and pharynx. Though they have an intestine, it lacks a distal opening and branch into two ceca (pouch-like structures connected to the intestine).
With the exception of blood flukes, trematodes are hermaphroditic. Generally, each individual possesses male reproductive organs (two testes, a sperm duct, cirrus, prostatic gland, and seminal vesicle) as well as female reproductive organs (the ovary, oviduct, uterus, seminal receptacle, and vitelline glands among a few others).
Once the eggs are released into the environment (river, lake) by the definitive host, they develop into miracidium and penetrate the intermediate host (snail or some aquatic animals), and transform into sporocytes. If the intermediate host is consumed raw by the definitive host, the life cycle continues.
Germinal cells in the sporocyte might develop giving rise to redia which is again released into the environment (water) when the sporocyte raptures. In the water, they develop into cercaria which attaches to vegetation and transform into metacercaria. If ingested by animals like sheep, the metacercaria develops into an adult fluke within the intestinal tract of the animal and invades the liver.
* Sexual reproduction in trematodes generally occurs in the adult stage of their life cycle. This takes place in the definitive host. As well, asexual reproduction occurs in the first intermediate host.
* Trematodes are responsible for a number of diseases in human beings with schistosomiasis being the most common. The disease results in organ damage and can be fatal if untreated.
The following is a diagrammatic representation of a fluke:
Some species of the Class Trematoda include:
All Platyhelminthes are commonly known as flatworms. They are characterized by a dorsoventrally flattened body with bilateral symmetry. This is largely attributed to the fact that they lack a coelom (they are mostly acoelomate).
They are triploblastic which means that their embryos have three germ layers (ectoderm, the endoderm, and the mesoderm). The ectoderm gives rise to the nerve cells and epidermis while the mesoderm gives rise to the muscles and structures of the reproductive system. Lastly, the endoderm produces the digestive organs in some of the species.
Since they are acoelomates, Platyhelminthes do not have any cavity between the intestinal region and the body wall. The digestive system in most flatworms is very simple or absent altogether.
Some of the species possess cilia at some stage of their life cycle. In turbellarians (planarians), for instance, cilia originate from the ventral epidermis and facilitate locomotion.
They consist of 9+2 anoneme as well as the inner and outer dynein motors that promote movement. Located on the ventral side, these structures are in contact with substrates which allow the organism to crawl/creep across the surface.
Aside from locomotion, cilia are also involved in excretion. In planarians (and other flatworms) cilia are associated with flame cells and the excretory pore. Here, the beating action of cilia helps direct the waste material into the tubules through which they are removed from the body.
Generally, the excretory system consists of tubules across the body with pores that open to the outside. These tubules are also connected to flame cells (function like kidneys) located on the ventral side of the organism. As the cilia continue beating, waste material from these cells (flame cells) are accumulated into the tubules before they are pushed to the excretory pore to be excreted.
* Some Platyhelminthes larvae also possess cilia for free-swimming.
Respiration - Unlike higher animals, flatworms do not have a respiratory system. As a result, respiration occurs through the body surface/cutaneous respiration (through simple diffusion). This involves the diffusion of oxygen across the epidermis to reach the cells and the movement of carbon dioxide from the cells to the environment through the same epidermis.
Unlike most animals, flatworms also lack a circulatory system. Therefore, oxygen diffusing through the skin moves directly to the cells and vice versa with carbon dioxide.
Organ level of organization - Despite being simpler animals, as compared to higher ones, flatworms have an organ level of organization. As mentioned, they are triploblastic and the three germ layers give rise to various tissues and organs. For instance, most of the species have well-developed reproductive organs that allow them to reproduce sexually.
Digestive tract - The digestive system of Platyhelminthes is very primitive or lacking. Tapeworms, for instance, do not have a digestive tract. Rather, nutrients, from the intestine of the host, are absorbed directly through the tegument.
In some of the other Platyhelminthes, however, the mouth part (pharynx) leads to an opening that branches into a sac-like cavity known as ceca. In turbellarians, undigested materials are again removed from through the mouth.
Parenchyma - Though they do not have body cavities, the spaces between the body wall and the organs are filled with parenchyma tissue. In turbellarians, for instance, all the free spaces are occupied by this connective tissue.
Unlike most animals, the parenchyma in Platyhelminthes contains very little or no extracellular matrix. However, it contains several types of cells including stem cells, true parenchyma cells, as well as the in-sunk bodies of epithelial cells. Whereas the in-sunk bodies originate from some of the other organs (e.g. the reproductive organs), stem cells are mostly neoblasts (prulipotent cells).
The parenchyma serves several functions which include:
Nervous system - The nervous system of Platyhelminthes consists of a central nervous system with a small, simple brain and two nerve cords as well as a peripheral nervous system with nerves extending throughout the body.
The brain of planarians is located in the anterior portion of the body and comprises two cephalic ganglia connected by a commissure. The longitudinal nerve cords extend from the brain and are connected to commissures and small lateral nerves. Lateral nerves give rise to the peripheral nerve plexuses.
Platyhelminthes also possess sensory elements that are connected to the brain. These include statocysts (involved in balance and orientation), photoreceptors (sense light intensity), epidermal sensory receptors (respond to touch and changes in environmental conditions), as well as light refractile bodies (containing small granules).
Collins, J. (2017). Platyhelminthes. Current Biology.
Noreña, C., Damborenea, C., and Brusa, F. (2015). Phylum Platyhelminthes. Ecology and General Biology, Fourth Edition, 2015, 181-203.
Reed, P., Floyd, R., Klinger, R., and Petty, D. (2012). Monogenean Parasites of Fish. University of Florida.