Difference Between Platyhelminthes and Nematoda

Main Difference – Platyhelminthes vs Nematoda

Platyhelminthes and Nematoda are two animal phyla, consisting of invertebrate worms. Though both phyla consist of worms, the two types of worms are not closely related. The main difference between Platyhelminthes and Nematoda is that Platyhelminthes consists of flatworms with a thin, dorso-ventrally flattened body whereas Nematoda consists of a cylindrical body, which is tapered to fine ends at each end. Tapeworms, Turbellaria, Trematoda, and Monogenea examples of Platyhelminthes. Secernentea, Enoplea, Chromadoria, and Adenophorea are the examples of the Nematoda. The flatworm, Planaria and the roundworm, Caenorhabditis elegans are the most studied worms in the lab.

Key Areas Covered

1. What are Platyhelminthes
     – Definition, Characteristics, Classification
2. What are Nematoda
     – Definition, Characteristics, Classification
3. What are the Similarities Between Platyhelminthes and Nematoda
     – Outline of Common Features
4. What is the Difference Between Platyhelminthes and Nematoda
     – Comparison of Key Differences

Key Terms: Digestive System, Flatworms, Invertebrates, Locomotion, Nematoda, Platyhelminthes, Roundworms

What are Platyhelminthes

Platyhelminthes refer to a phylum of invertebrates that comprises the flatworms. Around 20,000 Platyhelminthes species can be found on earth. Platyhelminthes are also called flatworms. Most flatworms are parasitic. Some are free-living in seawater or fresh water. Platyhelminths are triploblastic animals with bilateral symmetry. Their body is dorso-ventrally flattened. They are acoelomates since they lack a body cavity. The digestive system of Platyhelminthes is an incomplete digestive system. It lacks a mouth; the oral cavity directly opens into the pharynx. It also does not have a stomach. The gastrovascular cavity serves as the stomach. Some Platyhelminthes do not have a digestive system at all. A marine flatworm is shown in figure 1.

Figure 1: Pseudobiceros gloriosus

Respiration of Platyhelminthes occurs by simple diffusion through the body surface. Platyhelminthes lacks a circulatory system. The excretory system of Platyhelminthes consists of Protonephridia with Flame cells. Asexual reproduction of Platyhelminthes occurs either by regeneration or fission. Platyhelminthes are hermaphrodites with internal fertilization. Turbellaria, Trematoda, and Cestoda are the three classes of Platyhelminthes.

What are Nematoda

Nematoda refers to a phylum of invertebrates with cylindrical bodies. Around 15,000 species of nematodes have been identified in the world. They are also called roundworms. Since roundworms have a full-fledged body cavity, they are classified as pseudocoelomates. Nematodes lack a well-defined head. They have a simple digestive system. The mouth is at one end of the body. Nematodes consist of a hard outer covering called cuticle in their epidermis. A nematode is shown in figure 2.

Figure 2: A nematode

Nematodes consist of a life cycle with seven stages: egg, four larval stages, and two adult stages. Some nematodes are hermaphrodites, and others are dioecious.

Similarities Between Platyhelminthes and Nematoda

• Both Platyhelminthes and Nematoda belong to the kingdom Animalia.
• Some Platyhelminthes and Nematoda are free-living, and some are parasitic.
• Both Platyhelminthes and Nematoda are unsegmented worms.
• Both Platyhelminthes and Nematoda are multicellular, mobile, invertebrate animals.
• Both Platyhelminthes and Nematoda are heterotrophs.
• Both Platyhelminthes and Nematoda show bilateral symmetry.
• Both Platyhelminthes and Nematoda show organ level organization.
• Both Platyhelminthes and Nematoda are triploblastic animals.
• Both Platyhelminthes and Nematoda are protostomes.
• Both Platyhelminthes and Nematoda show spiral cleavage.
• Both Platyhelminthes and Nematoda show determinate cleavage.

Difference Between Platyhelminthes and Nematoda

Definition

Platyhelminthes: Platyhelminthes refers to a phylum of invertebrates that comprises flatworms.

Nematoda: Nematoda refers to a phylum of invertebrates with cylindrical bodies.

Other Names

Platyhelminthes: Platyhelminthes are called flatworms.

Nematoda: Nematoda is also called roundworms.

Classification

Platyhelminthes: Turbellaria, Trematoda, and Cestoda are the three classes of Platyhelminthes.

Nematoda: Adenophorea and Secernentea are the two classes of Nematoda.

Coelom

Platyhelminthes: Platyhelminthes is an acoelomate animal.

Nematoda: Nematoda is a pseudocoelomate animal.

Type of Protostomes

Platyhelminthes: Platyhelminthes belong to the superfamily Lophotrochozoa.

Nematoda: Nematoda belongs to the superfamily Ecdysozoa.

Body Structure

Platyhelminthes: Platyhelminthes consists of a flat body.

Nematoda: Nematoda consists of a cylindrical body tapered to a fine point at each end.

Outer Covering

Platyhelminthes: Platyhelminthes do not have a cuticle. The body of Platyhelminthes often contains cilia.

Nematoda: Nematoda consists of a hard outer covering called cuticle.

Digestive System

Platyhelminthes: Platyhelminthes consists of an incomplete digestive system.

Nematoda: Nematoda consists of a complete digestive system.

Locomotion

Platyhelminthes: Platyhelminthes exhibits a thrashing motion.

Nematoda: Nematoda exhibits a gliding locomotion.

Diseases

Platyhelminthes: Schistosomiasis, lung flukes, and liver flukes are diseases caused by Platyhelminthes. 

Nematoda: Ascariasis, hookworm disease, and trichuriasis are the diseases caused by Nematoda.

Conclusion

Platyhelminthes and Nematoda are two invertebrate animal phyla. Platyhelminthes consists of flatworms whereas Nematoda consists of roundworms. The main difference between Platyhelminthes and Nematoda is the anatomical structure of the body of each invertebrate.

Reference:

1.“Phylum Platyhelminthes: General Characteristics and Classification.” Online Biology Notes, 9 June 2017, Available here.
2.“What Are the Main Characteristics of Nematoda?” Animals – mom.Me, Available here.

Image Courtesy:

1. “Pseudobiceros hancockanus” By Jens Petersen – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “AKK 8-004 #3 Burrowing nematode 7880 (2) Tridate Khaithong photo” by Scot Nelson (CC BY-SA 2.0) via Flickr