With over 15,000 known species, the physical adaptations of roundworms have allowed the worms to survive and flourish in a variety of environments and habitats. Roundworms (also known as nematodes) exist as parasites or as free living organisms and play a role as decomposers that break down organic materials to be utilised by bacteria. Roundworms do not have circulatory or respiratory systems and therefore have adaptations that help with the distribution of food, liquids and gases.
Roundworms are characterised by a simple wormlike structure with a lack of features such as cilia or a well-defined head. They have an internal body cavity, called a pseudocoelom, which looks like a tube within a tube and runs the whole length of their bodies. This inner tube is the alimentary canal of the roundworm and extends from the mouth to the anus. The pseudocoelom contains the intestines and reproductive organs of the roundworms.
The body of a roundworm has an epidermis, or skin, composed of a mass of cellular material and nuclei without separate membranes. This skin secretes an outer cuticle which is thick, tough and flexible. This cuticle is moulted usually four times before the roundworm reaches the adult stage. The cuticle provides structural support and, along with longitudinal muscles, allows the roundworms to bend from side to side and to move in a thrashing manner. The cuticle is permeable by fluids and gases, thus allowing respiration to occur over the whole body. The adaptation of a hard and flexible yet permeable skin cuticle enables roundworms to maintain their internal fluids under high pressure.
Roundworms have a nervous system with circum-pharyngeal nerve rings, longitudinal nerves that run through the body to the digestive and reproductive organs. Shorter nerves extend to the mouth from the nerve rings. The muscle cells of nematodes branch towards the nerves and there are a series of nerve centres along the length of the roundworms. Two nerve cords serve to activate the muscles. The nerve cords relay sensory information with tactile, chemosensory and light sensitive receptors and help with movement.
The head of roundworms contains a few small sense organs and a pharynx where food is pulled in, crushed, and then moves to the gut cavity. Nutrients and wastes are spread throughout the body cavity by diffusion and regulated by an excretory canal or tubules on each side of the body. Nitrogen waste is expelled through special cells called rennette cells directly through the body wall. The digestive system of the roundworms includes a mouth with teeth, a gut, anus and a pharynx.
Most roundworms have separate sexes where the males utilise a specialised spine to inject sperm into the female's reproductive tract through a mid-body hole called a gonophore. Most roundworms lay eggs which can be highly resistant to adverse environments such as dry, hot or cold conditions. Roundworms lay up to 27 million eggs at a time.
Every individual of the roundworm species has the exact same number of cells. This is called "eutely." Growth in roundworms is by an increase in the size of the cells instead of by an increase in the number of cells.