Roundworms have an alimentary canal, meaning they have a tube containing specialized digestive organs and they have two external openings. A mouth and an anus. This is sometimes referred to as a one-way traffic digestive system. Roundworms were among the first organisms to have two external, body openings. Their digestive system includes a mouth, pharynx, intestine, rectum and an anus. Alimentary canal extends from the mouth on the anterior end, to the anus located near the tail. Basically a Nematode is a long hollow tube within which is another tube, the alimentary canal and the reproductive organs. Nematodes are round in cross section, this is because unlike the other worms that below them in the phyla table they maintain their body fluids under great pressure (on average internal pressure in a nematode equals 70mm of mercury or 1.49 PSI, with a maximum recorded value of 125mm of mercury or 2.41 PSI). To contain this high pressure nematodes have an extremely tough, yet elastic and flexible cuticle. This cuticle consists of up to 9 layers of proteinaceous fibres, with 3 layers being easily discerned, these are called, from the outside in, the cortex, the matrix layer and the fibre layer. Despite its complexity the Nematode cuticle is permeable to both water and gases, so respiration occurs through it. Beneath the cuticle is a hypodermis and a layer of longitudinal muscle. The combination of the flexure of these muscles with the high pressure of the system produces a characteristic whip-like wriggle that Nematodes use to swim. Scientifically this is called undulatory propulsion with sinusoidal waves passing back along the body.