What is the Difference Between Cestodes and Nematodes

The main difference between cestodes and nematodes is that cestodes are tapeworms that belong to the phylum Platyhelminthes, whereas nematodes are roundworms that belong to the phylum Nematoda. Furthermore, cestodes have a long, ribbon-like, dorsoventrally flattened body while nematodes have a cylindrical body tapered at each end. Moreover, cestodes are acoelomates while nematodes are pseudocoelomates. 

Cestodes and nematodes are two types of parasitic worms or helminths in the intestine. Generally, both types are endoparasites. 

Key Areas Covered 

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

Key Terms 

Acoelomate, Cestodes, Helminths, Nematodes, Platyhelminthes, Pseudocoelomate, Tapeworms

Cestodes – Definition, Characteristics, Behavior 

Cestodes or tapeworms are a class of parasitic worms that belong to the class Cestoda under the phylum Platyhelminthes. Generally, they have a flat, ribbon-like body with a single anterior holdfast organ called scolex and numerous segments called proglottids. Also, they do not have a digestive system, and therefore, they directly absorb nutrients through the tegument, which is the neodermal cuticle. In addition to these, gas exchange takes place through the tegument.

Figure 1: Cestoda Life Cycle

Furthermore, cestodes are acoelomates without a body cavity. Significantly, their flat body facilitates the perfusion of all tissues. On the other hand, cestodes are hermaphrodites; each segment of the body contain both male and female sex organs. Some of the examples of cestodes include Taenia solium (pig tapeworm), Taenia saginata (beef tapeworm), Diphyllobothrium latum (fish tapeworm), and Echinococcus granulosus (dog tapeworm). 

Nematodes – Definition, Characteristics, Behavior 

Nematodes or roundworms are a diverse group of worms that belong to the phylum Nematoda. Generally, they live freely in freshwater, saltwater, and soil while others are parasitic in animals, including vertebrates and plants. For instance, nematodes have a long thin unsegmented tube-like body. Also, they are pseudocoelomates with a fluid-filled body cavity, acting as a hydrostatic skeleton providing rigidity.

Figure 2: Nematoda Life Cycle

Moreover, nematodes have an anterior mouth and a longitudinal digestive tract. Significantly, in the head, they have lips, teeth, filariform extremities, and dentary plates for both attachment and sucking liquids from walls of the digestive system of the host. In the meanwhile, their digestive system lacks a stomach. On the other hand, the body of nematodes is covered by a thick, collagenous cuticle. Basically, some of the examples of parasitic roundworms are ascarids (Ascaris), filarias, hookworms, pinworms (Enterobius), and whipworms (Trichuris trichiura).   

Similarities Between Cestodes and Nematodes 

• Cestodes and nematodes are two types of helminths or parasitic worms of the intestine. 
• They are multicellular mobile invertebrates whose adults are visible to the naked eye. 
• They are triploblastic animals with bilateral symmetry. 
• Also, they are protostomes with spiral, determinate cleavage. 
• Both are endoparasites, requiring a host for the completion of their life cycle.  
• Besides, they feed on sucking liquids and have specialized mouthparts for the function. 

Difference Between Cestodes and Nematodes 

Definition 

Cestodes refer to a class of parasitic worms that are ribbon-like, multisegmented and dwell as adults entirely in the human small intestine while nematodes refer to another class of worms with elongates, a cylindrical body and either free-living or parasitic in animals and plants. 

Taxonomy 

Cestodes belong to the phylum Platyhelminthes while nematodes belong to the phylum Nematoda. 

Also Known as 

Another name for cestodes is tapeworms while another name for nematodes is roundworms. 

Number of Species 

Around 6000 species of cestodes are there while around 25,000 species of nematodes have been identified so far. 

Type of Protostomes 

Cestodes belong to the superfamily Lophotrochozoa while nematodes belong to the superfamily Ecdysozoa.  

Coelom 

Cestodes are acoelomates while nematodes are pseudocoelomates. 

Size 

Cestodes can grow up to 30 m while nematodes can grow up to 1 m. 

Shape 

Cestodes have a long, ribbon-like, dorsoventrally flattened body while nematodes have a cylindrical body tapered at each end.  

Cuticle 

Cestodes have a neodermal cuticle called tegument while nematodes have a thick, collagenous cuticle. 

Segments 

Cestodes are multisegmented worms while nematodes are unsegmented worms. 

Mouthparts Anatomy 

Cestodes have the scolex, is typically a four-sided knob, armed with suckers or hooks or both while nematodes have either three or six lips, which often bear a series of teeth on their inner edges.  

Mouthparts Function 

Mouthparts of cestodes help to attach to the intestine of the definitive host while the mouthparts of nematodes are responsible for sucking liquids. 

Digestive System 

Cestodes lack a digestive system and directly absorb nutrients through the body wall while nematodes have a digestive system with two ends. 

Locomotion 

Cestodes show a trashing motion while nematodes show a gliding motion. 

Reproduction 

Cestodes are hermaphrodites, reproducing through cross-fertilization while nematodes are dioecious, reproducing through copulation. 

Parasitic Species 

Around 40 species of cestodes are parasitic while around 12,000 species of nematodes are parasitic. 

Pathological Conditions 

Cestodes cause tapeworm infection while nematodes cause filariasis, elephantiasis, ascariasis, hookworm infection, etc. 

Transmission 

Cestodes transmit to the definitive host through the consumption of contaminated parts of the intermediate host while nematodes transmit through the infected fecal matter or consuming animal tissue with larva.  

Pathology 

Weight loss, tiredness, diarrhea, and vomiting are the symptoms of tapeworm infection while nematodes cause infections in the lungs. 

Diagnosis 

Cestodes segments can be identified in the feces while nematodes can be revealed by the fecal flotation exam.   

Examples 

Examples of cestodes include Taenia solium, Taenia saginata, Hymenolepis spp., Echinococcus granulosus, Multiceps multiceps, etc. while the examples of nematodes include Ascaris spp., Onchocerca spp., Rhabditis spp., Trichuris spp., Necator americanus, Ancylostoma spp. etc. 

Conclusion 

Cestodes are the tapeworms with a dorsoventrally flat body. Therefore, they belong to the phylum Platyhelminthes. Significantly, cestodes do not have a digestive system, but, they directly absorb nutrients from the intestine of the host. In contrast, nematodes are roundworms of the phylum Nematoda. Generally, they have a cylindrical body. However, most of the nematodes are free-living while some are parasitic. Usually, those parasitic nematodes have sucking mouthparts to absorb food from the host. Also, they have a digestive system with two ends. Therefore, the main difference between cestodes and nematodes is their anatomy and type of parasitism. 

References:

1. Castro GA. Helminths: Structure, Classification, Growth, and Development. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 86.Available Here.

Image Courtesy:

1. “Taenia solium Life cycle” By CDC/Alexander J. da Silva, PhD/Melanie Moser, Courtesy: Public Health Image Library (Public Domain) via Commons Wikimedia   
2. “Ascaris lumbricoides life cycle” By SuSanA Secretariat (CC BY 2.0) via Commons Wikimedia