Class of nematodes and their structure

Nematodes (from the Greek. Nemas – “thread”) are a class of Roundworms (about 20 thousand species). Free-living nematodes live on the bottom of seas and fresh water bodies, in damp earth and leaf litter (they are very small – from 0.05 to 50 mm).
Parasitic nematodes live inside other animals and humans, reaching a length of 20–40 cm, such as roundworm, for example.

Musculocutaneous sac and primary body cavity

The body of nematodes is fusiform: it is narrowed towards the anterior and posterior ends.

A layer of epithelial cells lies under a dense elastic membrane – the cuticle. Below are the longitudinal muscles – four single-layer ribbons. This structure allows the roundworms to crawl, bending the body.
The cuticle, epithelial cells and muscles form the skin-muscle sac.
The primary body cavity is located between the skin-muscle sac and the intestine. It is filled with liquid, which, due to pressure, maintains a constant body shape, promotes the distribution of nutrients throughout the animal’s body, and the movement of decay products to the excretory organs.

Digestive system

Unlike ciliated worms and flukes, nematodes have an anal (anal) opening, and the intestine has the form of a straight tube passing through the whole body.

The mouth opening is at the front end of the body and is surrounded by the lips. The anterior part of the intestine, the pharynx, has dense, muscular walls. Free-living nematodes feed on bacteria, algae, organic particles, rotted organic residues – detritus. But there are also predators, parasites of fungi, plants and animals. Some have cuticle outgrowths in the throat – peculiar teeth. With their help, nematodes pierce the integuments of animals and plants.

Gas exchange and metabolism

In free-living roundworms, gas exchange (oxygen consumption and carbon dioxide emission) occurs through the integument of the body.
In parasitic nematodes, the energy necessary for vital activity is released due to the breakdown of organic matter accumulated in the body – glycogen.
Due to the lack of a circulatory system, nutrients and waste products in the body are carried by the fluid in the primary body cavity.

Excretory system

The excretory system consists of two lateral blindly closed channels. They open outward with an excretory opening on the ventral side of the front of the body. The canal walls are formed by one or more very long cells (their length can reach 40 cm). The harmful substances formed in the body enter the cavity fluid, then into the channels of the excretory system, and are removed outside.

Nervous system

The nervous system of nematodes is represented by the periopharyngeal nerve ring and longitudinal nerve trunks – a stem-type system.

The sense organs in roundworms, especially in parasitic species, are very poorly developed. On the front of the body there are setae, which serve as organs of touch, and the olfactory pits. Some free-living nematodes have primitive eyes.


Nematodes are dioecious animals. The genitals have the shape of tubes: in females they are paired, in males they are unpaired.
Females have paired ovaries, oviducts, uterus and unpaired genital opening, which opens on the ventral side of the body.
The male has one filamentous testis, gradually turning into a larger vas deferens.

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