Difference Between Nematodes and Annelids

Nematodes vs Annelids

Both Nematodes and Annelids are invertebrates with elongated bodies, and they prominently feature among worms. Although they both are very important animals for humans in many ways, the knowledge about Nematodes and Annelids is much low among people. Therefore, the majority of people would not know how to distinguish these animals from each other. Hence, the need to understand their characteristics is high, and this article would help a lot for that.


Nematodes, the members of the Phylum: Nematoda, are also known as Round worms. There are about one million nematode species according to some of the estimations, and already there are 28,000 have been described. The majority of the Nematodes (16,000 species) are parasitic, and that is the reason for the notoriety about the round worms. The largest member of the phylum is about five centimetres long, but the average length is about 2.5 millimetres. The smallest species cannot be observed unless there is the aid of a microscope. Nematodes have complete digestive systems with the mouth at one end of the body while anus located at the other end. The mouth is equipped with three lips, but sometimes the number of lips could be six, as well. They are not segmented worms, but the anterior and posterior ends are tapered or narrowed. However, there are few ornaments viz. warts, bristles, rings, and other small structures. The body cavity of Nematodes is a pseudo coelom, which is lined with mesodermal and endodermal cell layers. The cephalisation or the formation of head to be distinctive from the other body parts is not prominent among Nematodes, but they do have a head with nerve centres. The parasitic species have especially developed some nerve bristles to sense the environment that they live.


Annelids are a large phylum consisting segmented worms, rag worms, earthworms, and nuisance leeches. There are over 17,000 extant species of Annelids presently. Usually, they live in freshwater or saltwater as well as around moist terrestrial environments. The body of an Annelid is elongated with segmentation has taken place externally through ring-like constrictions. These constrictions are called as annuli, and they are internally segmented or partitioned via septa at the same places as the annuli. Their segmentation could be regarded as the first sign of differentiation of body parts into different functions. Annelids secrete their cuticle from their skin cells, and the cuticle is made up of collagen but it is not as hard as in many other invertebrates. Many scientists believe that they have capillaries to take blood through organs, and it is a closed circulatory system in Annelids. They usually do not moult their cuticle, but some species shed their skin (leeches) or jaws (polychaetes). Their body cavity is a coelom but some Annelid species do not have a coelom and some have it in a very small place. Their true coelom is lined with mesodermal tissues, and this is the first instance in the evolutionary sequence where a true body coelom is found. The presence of parapodia in Annalids is an excellent adaptation to move in the environment.


What is the difference between Nematodes and Annelids?

• Nematodes have been more diversified than Annelids.

• Annelids have larger bodies compared to Nematodes.

• Annelids have segmented bodies without tapered ends, whereas the bodies of Nematodes are tapered at both ends but not segmented.

• Majority of Nematodes are parasitic but not the Annelids.

• Body cavity of Annelids is a true coelom while it is a pseudo coelom in Nematodes.

• Annelids have parapodia to walk but not the Nematodes.

  • vishal

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