What Features Distinguish Annelids from Roundworms

Annelids are segmented worms whereas roundworms are not segmented. Further, annelids have a true coelom while roundworms have a pseudocoelom. This means annelids have a well-developed mesoderm while roundworms lack a mesoderm. These are the main features that distinguish annelids from roundworms.

Annelids and roundworms are invertebrates with elongated bodies. Both exhibit bilateral symmetry. They have a complete digestive system.  

Key Areas Covered 

1. Annelids
     – Definition, Taxonomy, Characteristics
2. Roundworms
     – Definition, Taxonomy, Characteristics
3. What are the Similarities Between Annelids and Roundworms
     – Outline of Common Features
4. What Features Distinguish Annelids from Roundworms
     – Comparison of Key Differences

Key Terms

Annelids, Body Cavity, Roundworms, Segmentation

Difference Between Roundworms and Annelida - Comparison Summary

Annelids – Definition, Taxonomy, Characteristics 

Annelids are segmented worms that belong to the phylum Annelida. They are the most complex form of worms. Most annelids are beneficial but, some can be parasitic.

What Features Distinguish Annelids from Roundworms

Figure 1: An annelid

The three classes of annelids are Polychaeta (bristle worms), Oligochaeta (earthworms), and Hirudinea (leeches). Bristle worms are the largest group of annelids that mostly live in marine habitats. Earthworms are terrestrial members. Leeches are parasitic members of annelids.

Roundworms – Definition, Taxonomy, Characteristics 

Roundworms are cylindrical worms whose body is tapered at both ends. They belong to the phylum Nematoda. They are mostly parasitic, living inside plants and animals. On that account, their body surface consists of ridges, warts, and rings.

Distinguishing Features - Annelids and Roundworms

Figure 2: Roundworms  

The three classes of roundworms are Enoplea (enopleans), Dorylaimida (dorylaims), and Secernentea (secernenteans). Dorylaims live in soil or freshwater. Both enopleans and Secernenteans are parasites of plants and animals including humans.  

Similarities Between Annelids and Roundworms 

  • Annelids and roundworms are invertebrates with an elongated body. 
  • They have a bilateral symmetry. 
  • Both have a complete digestive system. 
  • They can be either free-living or parasites in plants and animals. 

What Features Distinguish Annelids from Roundworms

  1. Segmentation – Annelids have a segmented body while the body of roundworms is not segmented. These segments are the repeating sections that collectively form the body of annelids. Each section consists of a separate set of internal organs. However, annelids have a blunt-ended body while roundworms have tapered ends in their body. 

  2. Mesoderm and Coelom – Annelids have a well-developed mesoderm while roundworms do not have a mesoderm. So, annelids may have a true coelom while roundworms may have a pseudocoelom. In addition, the organs of the annelids are separated from the coelomic fluid, while in roundworms, organs swim in the liquid of the coelom. 

  3. Body Structure – Annelids are worms with the most complex body plan while roundworms have a medium complex body. Also, annelids are 8-10 inches long while roundworms can be over 4 feet long. Furthermore, annelids have a complete circulatory system with a heart, blood vessels, and blood while roundworms do not have a circulatory system and the nutrients move through diffusion. Moreover, annelids have both longitudinal muscles and circular muscles while roundworms only have longitudinal muscles. However, annelids have eyespots while roundworms have more complex eyes than annelids. Also, roundworms have a rigid cuticle made up of collagen in their epidermis.

  4. Feeding – Most annelids live inside the soil. They feed on primary detritus and other decaying organic matter. Also, these annelids help in the aeration of the soil. On the other hand, most roundworms are parasites in plants and animals including humans. 

  5. Reproduction – Generally, annelids are hermaphrodites while most roundworms are dioecious animals with separate sexes. But, some roundworms are hermaphrodites as well. 


Annelids are the most complex worms with a segmented body. On the other hand, roundworms have a cylindrical body with tapered ends. Most annelids are beneficial while roundworms are infectious.  These are the features that distinguish annelids from roundworms.


1. “Worms: Phyla Platyhelmintes, Nematoda, and Annelida.” Exploring Our Fluid Earth, Kaunana, Available Here  

Image Courtesy:

1. “Annelid worm, Atlantic forest, northern littoral of Bahia, Brazil (16107326533)” By Alex Popovkin, Bahia, Brazil  (CC BY 2.0) via Commons Wikimedia   
2. “Toxocara-canis-adults” By Alan R Walker – Own work (CC BY-SA 3.0) via Commons Wikimedia

About the Author: Lakna

Lakna, a graduate in Molecular Biology and Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things. She has a keen interest in writing articles regarding science.

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