Hobbies And Interests
Cuticle
Roundworms have an unsegmented exterior that is unusual in that it is not a collection of cells like human skin. The cuticle that covers roundworms is a collection of cellular secretions; it is similar to the covering of arthropods, such as insects and crustaceans, though, more flexible. The proteinaceous and lipid composition of the cuticle protects the parasitic species from the digestive enzymes of their hosts.
Muscles
While the roundworms are radially symmetrical, their muscles do not form rings. Longitudinal muscles run from one end to the other, allowing for movement through "dorso-ventral undulation," according to the University of California at Davis. Like the bone and muscle action in humans, roundworms use hydrostatic pressure to act against the contractions of their muscles.
Neural Ring
Roundworms have a neural ring connected to nerve cords along the back and belly and sensory neurons in the head. These are also connected to motor neurons that cause muscular contraction, though, unlike most animals, the muscles branch into the neurons rather than the other way around.
Digestive System
Roundworms are the lowest form of life to develop a "tube within a tube"-style digestive system, not unlike our own although theirs is much more rudimentary. A distinguishing characteristic of nematodes is the beginning of their alimentary canal. Triradiate mouthparts, or lips, open onto a pharynx, where food is crushed. As food passes through the linear alimentary canal, nutrients are extracted, and waste is excreted through the animal's anus -- where the digestive tract terminates.
Respiratory System
Roundworms lack a respiratory or circulatory system. Gas exchange takes place through diffusion across the cuticle layer, and osmoregulation -- the balance between salt and water performed by the kidneys in humans -- occurs through the excretory canals running along the worm's sides.