Phylum Aschelminthes or Nementhelminths- The Round Worms

Gegenbaur (1859) proposed the term 'Nementhelminthes' . The phylum Nementhelminthes includes about 15,000 species.

GENERAL CHARACTERS

1. Habitat- Many roundworms live as parasites in plants and animals. They cause serious agricultural, veterinary and human health problems. Round worms are also free living and occur in freshwater, sea water and soil.
2. Body Form - They are called roundworms because they appear circular in cross section. They are unsegmented.
3. Body Wall- It consists of firm, non-living resistant cuticle, syncytial epidermis and muscle layer.
4. Body Cavity- The body cavity is called pseudocoelom because mesoderm is found as scattered pouches inbetween the ectoderm and endoderm.
5. Digestive tract- Alimentary canal is complete with muscular pharynx.
6. Skeletal, Respiratory and Circulatory systems are absent. Pseudocoelomic fluid pseudocoelom maintains body shape and forms hydroskeleton. Gaseous exchange is aerobic respiration occurs by diffusion through the body surface. The pseudocoelomic fluid transports materials.
7. Excretory system- The excretory system consists of gland cells. In Ascarias, 'H' shaped excretory system of canals and complicated 'giant cells' called 'renette cell' is present.
8. Nervous system- It consists of Circumpharyngeal ring that gives rise to nerves forward as well as backward.
9. Sexes- Sexes are separate (dioecious) Generally they show sexual dimorphism , often females are longer than males. Fertilization is internal.
10. Development- Development may be direct or indirect. During indirect development a larvae is present. Filariform larvae is present in Ancyclostoma, microfilaria larvae is present in Wuchereria and Rhabdiform larvae is present in Ascaris and enterobius.
    

Classification

Aschelminthes are classified into two classes 

Class 1. Aphasmidia: (i) Phasmids are absent    (ii) Amphids are of various types , e.g,. Trichinella ,Trichuris etc.

Class 2. Phasmidia: (i) phasmids are present near hind end of the body. (ii) Amphids are present near anterior end, e.g., Ascaris, Enterobius, Ancyclostoma, Wuchereria etc.

Key points 

• Dual excretory waste: Ascaris is both Ammonotelic and ureotelic.
• Monorchic: Presence of single testes, e.g., male Ascaris.
• Didelphic: Presence of two uteri, e.g, Female ascaris.

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