Roundworms (nematodes) are invertebrate animals characterized by a large species diversity. During evolution, roundworms appeared after flatworms and have more complex structure and functions compared to them.
Roundworms - Cavities
The body of roundworms has the shape of a spindle, and in cross section it is round. Hence the name of the type. The body of roundworms is not divided into segments.
Evolutionary neoplasm is the primary cavity of the body, or pseudo-goal. The pseudocele is filled with intercellular fluid, internal organs are located in it. The liquid serves as a hydroskeleton, gives the body elasticity and facilitates the metabolism between organs.
The body of roundworms consists of three layers. The upper layer is called the cuticle, it acts as the outer skeleton. The cuticle also protects the body from damage.
The second layer is composed of epithelial cells (hypodermis), metabolic processes occur here. Inside the third layer merges with hypodermis - muscle cells.
The muscles of roundworms are smooth. There are four longitudinal single-layer muscle tapes. They allow roundworms to crawl, bending the body.
Due to the smooth muscles, roundworms can move very quickly and energetically. For example, large nematodes can make their way into rather narrow openings.
Differentiated organ systems of roundworms
In total, roundworms have five organ systems. Only the circulatory and respiratory systems are absent. During 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. In them, insoluble metabolic products and foreign bodies penetrate 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 genital organs, including the testis and vas deferens.
The nervous system of roundworms is a near-pharyngeal nerve ring and six nerve trunks. Nerve trunks are connected by jumpers. Roundworms have tactile tubercles and chemical sense organs as sensory organs.
