Triploblastic Animals – Pseudocoelomates – Aschelminthes (phylum Nematoda) – The Round worms

General characteristics: The name nematode means “pointed ends”. The animals included in this group have elongated worm like body with pointed ends. The nematodes are triploblastic and pseudocoelomates. One end of the body is anterior, however the head is not clearly marked and there are no special sense organs at this end. The nematodes exhibit bilateral symmetry and the body is unsegmented. The body cavity is pseudocoelom.

It is derived from the hollow space, the blastocoels, situated in the blastula, an early stage in embryological development, and not from the mesoderm. It consists of a number of vacuolated cells filled with a protein-rich fluid which develops high hydrostatic pressure.

The nematodes range from small microscopic forms, to some from reaching a length of up to one meter. The digestive system is in the form of alimentary canal with two openings. The opening at the anterior end is mouth and at the posterior end is the anus. In parasitic nematodes the digestive system is simple. A fluid filled space is present between the body wall and alimentary canal. It provides “tube within tube” type structure in nematodes.

The excretory system consists of two longitudinally running excretory through an excretory pore on the ventral surface. There is a nerve ring around the pharynx, which give rise to dorsal, ventral and lateral nerve cords running throughout the length of the worms. The sense organs are in the form of sensory papillae present on the lips at the anterior end. The circulatory and respiratory systems are absent. The gaseous


Exchange takes Pl through general body surface; locomotion is by undulating waves of contraction and relaxation of muscles. These muscles are arranged in four bands, two dorso-lateral and two ventro-lateral. The circular muscles are absent; therefore the bending is dorso-ventral only.

The sexes are separate. The female gonads are ovaries and these male gonads are testes which produce sperms. A larval stage is present in the life cycle.

Importance –Parasitic Diseases

Aschelminthes is important from the point of view of its parasites of which it has a great variety causing some very serious diseases in man and plants.

Ascaris dumbricoides is an intestinal parasite of man.

The genus Rhabditis contains numerous species normally found in soil, organic matter or water and feces of man or animals. Enterobius vermicularis commonly known as pin worm is cosmopolitan but more common in Europe and America. Pinworms are parasites in the human caecum, colon and appendix. Their movement causes intense itching of anus, inflammation of mucous membrane of colon and appendix resulting in insomnia and loss of appetite.

Ancylostoma duodenale is commonly known as hook worm. It is a parasite of human small intestine in Asia, North Africa and Europe. It is very dangerous because it holds the villi of intestine and sucks blood and body fluid. During feeding they produce an anticoagulant to prevent clotting of blood and after feeding leave the wound bleeding in children it can cause severe anemia and retard physical and mental growth.

  • Round worms are everywhere outdoors, where they play an important role in breaking down organic matter. A single rotting apple may contain 90,000 worms. Billions thrive in each acre of topsoil.

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