Phylum Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates that belongs to kingdom Animalia. This phylum contains 13,000 species and include many-living and parasitic life forms. They are acoelomates (have no body cavity), and have no specialized circulatory and respiratory organs. This restricts them to having flattened shapes so that oxygen and nutrients can pass through their bodies by diffusion. As a result, these organisms are also known as flatworms. The size of the members of this phylum range from a single-cell to around 2-3 feet long. This article covers characteristic features, classification and much more. Read on to know these exciting facts about the phylum Platyhelminthes.

Phylum Platyhelminthes: Definition, Examples

The simplest animals that are bilaterally symmetrical and triploblastic are the Platyhelminthes and commonly known as the flatworms.

Fig: Flatworm

History of Phylum Platyhelminthes

The term ‘Platyhelminthes’, which means flatworms, was coined by the scientist Gegenbaur. Modern Latin (plural), from ‘platy’, means ‘flat’ + Greek ‘helminth’ means ‘worm’. Scientists believe that the first turbellarians evolved around \(550\) million years ago. Scientists like Lamarck \((1816)\) separated between the flatworms and roundworms, Cuvier \((1817)\) included the flatworms within Zoophyta or Radiata, Vogt \((1851)\) called flatworms as Platyelmia. Platyhelminthes are mostly worm-like creatures that are dorsoventrally flattened, meaning they look like a ribbon and that why they are called Flatworm, Tapeworm, Fluke and Planarian.

What are the Characteristics of Phylum Platyhelminthes?

The members of the phylum Platyhelminthes are commonly known as flatworms. They exhibit the following characteristics:

1. They are-living, ectoparasitic and endoparasitic worms. Some are aquatic, and some have terrestrial habitats.
2. They show bilateral symmetry, and their body is unsegmented and dorsoventrally flat.
3. They are triploblastic with three germ layers, i.e., ectoderm, mesoderm and endoderm.
4. They are acoelomates; the body cavity is absent and shows organ grade organisation for the first time in the animal kingdom.
5. All the organ systems are packed in the parenchyma tissue, and they do not possess exoskeleton and endoskeleton. The parenchyma works as a hydraulic skeleton.
6. The body covering is soft and ciliated, rod-shaped bodies, the rhabditid are present in the epidermal cells of the living Platyhelminthes. They are used for protection and food capture.
7. The digestive system may be absent or present. If present, it is described as incomplete due to the absence of anus.
8. Respiration is by diffusion process through the body surface. Anaerobic respiration takes place in endoparasitic flatworms.
9. The excretion process is performed by a group of specialised cells called flame cells. They open into excretory canals. The excretory canal opens at the posterior end through the excretory pore.

Fig: Flame cell

10. The nervous system is ladder-like. It consists of a brain and two primary longitudinal nerve cords joined at intervals by transverse commissures. Sense organs are seen in-living flatworms.

11. These animals are usually hermaphrodite, and the reproductive organs are well developed. The fertilisation is always internal.

12. Anatomy of the body encourages cross-fertilisation rather than self-fertilisation. Asexual reproduction is by transverse binary fission in some animals.

13. Development is indirect in most flatworms with one or more larval stages. In the liver fluke miracidium, sporocyst, redia, cercaria and metacercaria larvae are present. In tapeworm, oncosphere, hexacanth and cysticercus larvae are found.

14. The regeneration process is well marked in some flatworms like Planaria.

Fig: Regeneration of Planaria

What is the Classification of Phylum Platyhelminthes?

Based on the mode of life, presence or absence of digestive system, cilia on the body wall, and the classification by Hyman, L.H., \((1951)\), the phylum Platyhelminthes is divided into three classes as follows:

Class \(1.\) Turbellaria

1. These are-living flatworms seen in both fresh and marine habitats.
2. Regeneration is common in these worms, for example, Planaria.
3. The mouth is ventral, and the pharynx is protrusible. 
4. Hooks and Suckers are absent.
5. Mostly hermaphrodite.
6. Examples: Bipalium, Planaria, Otoplana, Notoplana.

Class \(2.\) Trematoda

These animals are ectoparasitic or endoparasitic worms devoid of cilia.

1. The body is unsegmented and is covered with a cuticle, and bears one or more suckers.
2. The mouth is anterior in these class animals.
3. These are primarily hermaphrodite, but some are unisexual animals.
4. These have a single ovary, but testes can be more than two.
5. Some flukes of this class have a complicated life cycle.
6. Examples: Fasciola, Polystomum (endoparasite of the urinary bladder of frog and turtle), Clonorchis sinensis is Chinese liver fluke, Paragonium is a lung fluke.

Class \(3.\) Cestoda

1. These worms are endoparasitic in the alimentary canal of vertebrates.
2. The body is generally divided into few too many proglottids, i.e., not true segments.
3. In some flatworms, the anterior end is provided with adhesive structures like hooks and suckers.
4. The mouth and alimentary canal are absent, and each proglottid has one to two sets of complete hermaphrodite reproductive organs.
5. The life cycle is complicated and involves two or more hosts.
6. Examples: Taenia (pork tapeworm), Echinococcus (dog tapeworm), it is the smallest tapeworm, Taenia saginata is a beef tapeworm.

Phylum Platyhelminthes Example

Some of the examples of the phylum Platyhelminthes are as follows:

Taenia

Classification of Taenia–
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Cestoda
Order:  Cyclophyllidea
Family: Taeniidae
Genus: Taenia

Fig: Taenia

Planaria

Classification of Planaria–
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Rhabditophora
Order: Tricladida
Family: Planariidae
Genus: Planaria

Fig: Planaria

Fasciola

Classification of Fasciola–
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Rhabditophora
Order: Plagiorchiida
Family: Fasciolidae
Genus: Fasciola

Fig: Fasciola

Economic Importance of Phylum Platyhelminthes

The economic importance of Phylum Platyhelminthes are as follows:
i. Flatworms play a significant role in biological communities – marine, freshwater and terrestrial ecosystems.
ii. Flatworms absorb all oxygen via diffusion from the surrounding air.
iii. Most Turbellarians are carnivorous predators and scavengers and play a role in the food chain.
iv. Fasciola causes fascioliasis or liver rot, characterised as hepatitis.
v. Echinococcus causes hydatid disease, characterised by enlargement of the liver.

Interesting Miscellaneous Fact!

Adult tapeworms can measure more than \(80\) feet, i.e.,\(25\) meters long and can survive as long as \(30\) years in a host. They can also live in human intestines.

Fig: Tapeworm

Summary

In this article, we studied that Phylum Platyhelminthes are dorsoventrally flat animals having either unsegmented and leaf-like or ribbon-like bodies. They are the first animals to have bilateral symmetry and to undergo cephalisation. These animals have a high level of regeneration, and some are hermaphrodite. Under favourable conditions, they can infect the human body and cause multiple diseases. We also learned Phylum Platyhelminthes facts- habitats, their types, and their economic importance.

Frequently Asked Questions (FAQs) on Phylum Platyhelminthes

The most frequently raised doubts about Phylum Platyhelminthes are answered here:

Q.1. What is the common name for the phylum Platyhelminthes? Ans: The common name for phylum Platyhelminthes is flatworms.

Q.2. What are the \(3\) classes of the phylum Platyhelminthes? Ans: The \(3\) classes of the Phylum Platyhelminthes are Turbellaria, Trematoda and Cestoda.

Q.3. What animals belong to the phylum Platyhelminthes? Ans: The animals that belong to the phylum Platyhelminthes are Tapeworms, Liver fluke, Planaria, Fasciola, etc.

Q.4. What are the five characteristics of phylum Platyhelminthes? Ans: The five characteristics of the phylum Platyhelminthes are as follows: 1. They are-living worms, ectoparasitic and endoparasitic worms. Some are aquatic, and some have terrestrial habitats. 2. They show bilateral symmetry, and their body is unsegmented and dorsoventrally flat. 3. They are triploblastic with three germ layers, i.e., ectoderm, mesoderm and endoderm. 4. They are acoelomates with no body cavity. 5. They show organ grade organisation for the first time in the animal kingdom.

Q.5. Who discovered the phylum Platyhelminthes? Ans: Gegenbaur discovered the Phylum Platyhelminthes.

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