Nematodes, or ‘roundworms’, are totally different from platyhelminths and nemertines. A nematode is characterised by an extremely tough thick cuticle round the outside and a very high hydrostatic pressure within. They all look very similar (Figure 8.1a) and given the pressure it is hard to see how any other shape could be maintained, yet there are perhaps a million species. The phylum is unusually ubiquitous; nematodes are free-living in marine, freshwater and land habitats and parasitic in animals and plants. They are clearly of great economic importance as they exist in extraordinary numbers and play a significant role in the total matter and energy cycle of the biosphere. Buchsbaum (Animals Without Backbones, 1938) wrote, ‘If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognisable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes and oceans. The location of towns would be decipherable … Trees would stand in ghostly rows’, etc. We are told that four and a half million individual nematodes were found in one square metre of Dutch marine mud, and 90 000 in one rotten apple.

What makes these worms so remarkably widespread and numerous? The cuticle is clearly the answer, and it is argued below that the combination of tough cuticle and high internal pressure dictates most of their (often unique) characteristics.