Roundworms (nematodes) are invertebrates with a large species diversity. In the course of evolution, roundworms appeared after flatworms and have more complex structure and functions in comparison with them.
Roundworms - primary cavity animals
The body of roundworms is spindle-shaped, and in cross-section it is round. This is where the name of the type comes from. The body of the roundworms is not segmented.
An evolutionary neoplasm is the primary body cavity, or pseudo-goal. The pseudocoel is filled with intercellular fluid, and the internal organs are located in it. The fluid serves as a hydroskeleton, gives the body elasticity and facilitates the exchange of substances between organs.
The body of roundworms consists of three layers. The top layer is called the cuticle, which acts as the outer skeleton. The cuticle also protects the body from damage.
The second layer is made up of epithelial cells (hypodermis), metabolic processes take place here. From the inside, the third layer - muscle cells - merges with the hypodermis.
The musculature of roundworms is smooth. There are four longitudinal single-layer muscle bands in total. They allow the roundworms to crawl by bending their bodies.
Thanks to the presence of smooth muscles, roundworms can move very quickly and energetically. For example, large nematodes can wade into rather narrow openings.
Differentiated organ systems of roundworms
In total, roundworms have five organ systems. Only the circulatory and respiratory systems are missing. In the course of evolution, these systems appeared in annelids.
In free-living roundworms, gas exchange occurs through the surface of the body. In parasites, respiration is anaerobic.
The digestive system is represented by a through tube. At the front end of the body there is a mouth opening surrounded by lips. The digestive tube ends with the anus, which is also an evolutionary neoplasm.
The excretory system of roundworms includes cutaneous glands with an excretory duct.
Roundworms have special organs - phagocytic. They retain insoluble metabolic products and foreign bodies that enter the body.
As for the reproductive system, roundworms are dioecious. The female genitals are paired: ovaries, oviducts, uterus and genital opening. The male has unpaired genitals, including the testis and vas deferens.
The nervous system of roundworms is a periopharyngeal nerve ring and six nerve trunks. The nerve trunks are connected by jumpers. Roundworms have tactile hillocks and chemical sense organs as sense organs.
Where do roundworms live?
Roundworms live in a wide variety of environments. Some species live in the wild. They live in soil and in water (regardless of the salt content in it).
There are also types of roundworms that are parasites of living organisms. For example, pinworm, roundworm and trichina are common human parasites.
