Nematoda (round worms, hook worms, etc.)

Nematodes have/are

Nematodes don't have

Circular in cross-section A circulatory system
Bilaterally symmetrical and usually pointed at both ends Segmentation
Longitudinal muscles only, arranged in four zones  
A nervous system with four longitudinal nerve cords  
Complex cuticle  
Triangular arrangement of mouth  
Pseudocoelomate with body fluid always under high pressure (hydrostatic skeleton).  
Anterior mouth, muscular pharynx and gut, and anus  
Fertilisation is internal  
Parasitic and free-living  

Nematoda overview, Greek: nema = thread, eidos = form

They are everywhere! About 80,000 species of Nematode have been described so far, but some authorities estimate that there may be as many as 500,000 to 100 million species in all! They live in all environments and can parasitize nearly all animals and plants. In fact there is scarcely an animal alive that does not harbour a population of parasitic nematodes at some stage in its life, and that does include you! The soil-dwelling species tend to inhabit the top few centimetres of soil, and make thrashing movements when uncovered.

Around 50 different species are known parasites of humans (see below for more details). Luckily most of the time they are so harmless that we don't even know they are there.

Ubiquity To give you an idea of just how ubiquitous they are:

  • You could expect to find around 10 million in one square yard of soil, and 25 million in a square metre of oak-wood soil
  • One apple left on the ground to decompose had 90 000 individual nematodes living on and in it
  • One fig had 50 000 individuals belonging to over 8 species on and in it
  • In 1914 Nathan Cobb famously wrote "If all the matter in the universe except nematodes were swept away, our world would still be recognizable, its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes."

Parasitic species may be up to nine metres long, but most species are less than five centimetres long. The longest known nematode is found in the placenta of sperm whales.

Above right is a cross-section of a female Ascaris sp., and below right are a male and female ascaris inside a pig.

Ascaris sp. female nematode in cross section

The nematode body pattern

Compared to other phyla the external structure of the Nematoda is very uniform.

They all have slender, elongated bodies with tapered ends. Some have specialised mouthparts with hooks and stylets, and the only other projecting parts are concerned with reproduction.

The cuticle can have as many as nine layers, with cross fibres forming a spiral network. The cuticle is non-cellular and secreted by the epidermis, and is mainly collagen laid down in three criss-crossing layers.

The body is a high pressure pseudocoel within a cuticle covered body wall with only longitudinal muscles.

Locomotion is basically wriggling or thrashing with the longitudinal muscles on one side contracting, while the other side expands, deforming the body into S-shaped curves. The cuticle prohibits radial expansion, so serves the purpose of circular muscles.

The muscle arm extends to either the ventral or dorsal nerve cord. The pseudocoel functions as a hydrostatic skeleton. There are no circular muscles, and it is the body movement which moves food from the mouth to the pharynx, intestine, rectum and anus.

Males are usually smaller than females and the posterior is curled into a hook. This can be easily seen in the drawing on the right. This shows a male and female Ascaris sp. in the gut of a pig.

ascaris sp.

Crassicauda boopis, fin whale round worm

Parasitic nematodes in whales

Left is part of a whale's kidney infected with several Crassicauda boopis, the fin whale round worm. The head of the worm is found in the blood vessels in the liver, the middle in the whale's kidney, and the tail end in the reproductive and excretory system. The entire length of the worm can be as long as 8 m.

Below is a piece of a false-killer whale's gut which has a heavy infestation of Bolbosoma capitatum, the thorny-headed worm. There have been cases where infestations reach up to 600 worms per square metre of gut.

Bolbosoma capitatum, thorny-headed worm, in false-killer whale's gut

Mermis nigrescens, terrestrial nematode

Below is a terrestrial nematode Mermis nigrescens I found entwined around a poppy leaf after a heavy shower. It was 5 - 10 cm long, and the little red tip is at the head end.

It emerges after rain. and climbs on vegetation to lay its eggs. The eggs are eaten by earwigs and grasshoppers, and the nematode larvae hatch inside the host and thrive.

When the host is fully grown the nematode bursts out of the body cavity of the host killing it. While inside the host it has been coiled up, and on hatching it stretches out to several times the host's length. It takes up so much space, and absorbs so much nutrition that the host is usually not able to reproduce. Then it burrows into the soil and remains there for up to 2 years, before climbing vegetation after rain to lay its eggs. It is eaten by spiders and snails.

Terrestrial nematode

Dorylamius stagnalis, a freshwater nematode

Dorylamius stagnalis, a freshwater nematode

On the left is Dorylamius stagnalis, a freshwater nematode. It is found in mud, and among plant roots at the bottom of ponds. Its mouth has a stylets for piercing, and it grows to 5 - 8 mm long.

First animal to have its genome sequenced

One nematode, Caenorhabditis elegans, has become very well known as it was the very first multicellular organism to have its full genome sequenced. It is unusual in that it has a fixed number of cells (959) in its body.

Nematodes in bumblebees

Sphaerularia bombi

Sphaerularia bombi (above), a nematode infecting bumblebee queens. This parasite is only found in queens and affects her behaviour.

The bumblebee queen is infected by an adult female worm while she hibernates in the soil. It is believed that the nematode enters through her mouthparts. Around 12% of queens are infected, and this can rise to as much as 50% for late emerging queens.

In the spring when the queen emerges from hibernation the worm begins to grow, then it turns its whole reproductive system inside out. The uterus grows and grows till it is between 1-2 centimetres long, while the rest of the worm is only a thin thing of a few millimetres.

In a normal queen a hormone would be released and her ovaries would start to develop stimulating her to start building a nest, but somehow this does not happen in an infected queen.

Meanwhile the worm releases up to 100 000 eggs into the blood of the queen, these eggs hatch and develop, moving into the gut and reproductive system. During this time the queen feeds only for herself, she makes no attempt to find a nest site, and her ovaries do not develop. Often she returns to her hibernation site, here the worm larvae are discharged with faeces into the soil. The mature worms mate, and wait for another queen to use the site to hibernate.

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