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Phylum Aschelminthes - Characters and Classification | Animal Kingdom - What are the characteristics of Aschelminthes? - What is the phylum Annelida? - Why are Aschelminthes called roundworms? - What is the meaning of Nemathelminthes?

Phylum Aschelminthes Characters and Classification

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 fresh water, sea water and soil.

Body Form:
They are called round worms because they appear circular in cross section. They are un-segmented.
Body wall:
It consists of firm, non living resistant cuticle, syncytial epidermis (a continuous layer of cytoplasm with scattered nuclei) and muscle layer.
Body cavity:
The body cavity is called pseudocoel or pseudocoelom as it devel­ops from the blastocoel of the embryo. Pseudocoel is filled with pseudocoelomic fluid.
Digestive Tract:
Alimentary canal is complete with muscular pharynx.
 Skeletal, respiratory and circulatory systems are absent:
Pseudocoelomic fluid present in the pseudocoelom maintains body shape and forms hydro skeleton. Gas­eous exchange in aerobic respiration occurs by diffusion through the body surface. The pseudocoelomic fluid transports materials.
Excretory System
The excretory system consists of gland cells, or of canals or of both. In Ascaris, ‘H’ shaped excretory system of canals and complicated “giant cell” called “renette cell” is present. Ammonia is main excretory matter. However, Ascaris also excretes urea.
Nervous System:
It consists of a circumpharyngeal ring that gives rise to nerves forwards as well backwards.
Sense Organs:
(i) Papillae (raised structures) occur on the lips, on the sides of anterior end in both male and female and in front and behind the cloacal aperture. All papillae are tactile in function,
(ii) Amphids (pits) are present on the lips and are chemoreceptors.
(iii) Phasmids are unicellular glands located upon lateral sides of the posterior end and are glandulosensory in nature.
Sexes are separating (dioecious). Generally they show sexual dimorphism; often females are longer than males. Fertilization is internal. There is no asexual reproduction.
Development may be direct or indirect. During indirect development a larva is present. Filariform larva is present in Ancylostoma (hook worm), microfilaria larva is found in Wuchereria (filarial worm) and Rhabditiform larva is present in Ascaris and Enterobius (pin worm).
Unique Features:
(i) Syncytial epidermis,
(ii) Muscle layer of body wall consists of longitudinal fibres only,
(iii) Pseudocoel.
Advancement over Flatworms:
(i) Pseudocoel.
(ii) Complete digestive tract,
(iii) Uni­sexual condition.

Classification of Phylum Aschelminthes:

Aschelminthes is classified into two classes: Aphasmidia and Phasmidia.
Class 1. Aphasmidia:
(i) Phasmids are absent,
(ii) Amphids are of various types. Examples: Trichinella, (Trichina worm), Trichuris (Whip worm), etc.
Class 2. Phasmidia:
(i) Phasmids are present near hind end of body,
(ii) Amphids are present near anterior end.
Examples: Ascaris (Giant intestinal round worm), Enterobius (Pm worm), Ancylostoma (Hook worm), Wuchereria (Filarial worm), etc.
This genus includes free living and semi-parasitic forms. Rhabditis maupasi lives in soil. The male has copulatory bursa at the posterior end of the body.
Ascaris lumbricoides: The Giant Intestinal Roundworm.
Ascaris lumbricoides is an endoparasite of the small intestine of human beings. It is more common in children. The adult worms live for about 1 to 2 years. There is no secondary host in the life cycle to this parasite.
The mouth of Ascaris is bounded by three lips: one dorsal and two ventrolateral. The dorsal lip has two double papillae, while each ventrolateral lip has one double papilla, one simple lateral papilla and a amphid. Papillae are tactile (respond to touch).
Amphids are chemoreceptors (respond to chemicals). The animal shows sexual dimorphism. The phasmids are pair of unicellular glands that open outside on the lateral sides of the tail. These are glandulosensory in nature.Above mentioned points clearly indicate the sexual dimorphism in Ascaris.
It causes the disease, ascariasis. Since a large number of adult Ascaris worms normally infest a single host, they obstruct the intestinal passage and thereby cause abdominal discom­forts, like colic pains. The patient may also suffer from impaired digestion, diarrhea and vomiting.
In children, where the Ascaris infection is quite common, mental efficiency is affected and body growth is retarded. The disease can best be treated by administering antihelminthic drugs such as oil of Chenopodium, Alcopar, Bendex, Dewormis, Mebex, Pantel, Parid, Wormin, Zentel, etc.
Ancylostoma duodenale— The Hook worm:
It is found as an endoparasite in the small intestine of human beings. No intermediate host is required. Its larvae enter the human body from moist soil by boring through the skin of the feet. It causes ancylostomiasis disease.
Wuchereria (Filaria) — The Filarial worm:
Adult Wuchereria bancrofti lives as an endoparasite in the lymphatic vessels and lymph nodes of human beings (primary host). The mosquito (certain species of Culex) is the intermediate host. Thus its life cycle is digenetic it is viviparous.
In the lymph glands, the juveniles develop into adults. The accumulation of these worms blocks the lymphatic system resulting inflammation of lymph nodes. It causes a serious disease known as elephantiasis (= Filariasis) in the legs, arms, scrotum, etc.
Enterobius (= Oxyuris) — The Pin worm:
Enterobius vermicularis (vem- “Chunna”) is found in the caecum, colon or vermiform appendix of the human beings. Its life cycle is monogenetic.
Trichinella— The Trichina worm:
Adult Trichinella spiralis is found in the small intestine of human beings and some other mammals like pigs, domestic animals and rodents, however, its encysted larvae are present in the striated muscles of the host. It is viviparous. The disease caused by Trichinella spiralis is called trichinellosis.
Dracunculus— The Guinea worm:
Dracunculus medinensis occurs in the subcutane­ous tissue of man. When the host comes in contact with cold water, the female worm found below the skin, releases larvae in the water.
The larvae penetrate the body of Cyclops, a fresh water crustacean which is the intermediate host of the guinea worm. When the infected Cyclops is taken in by man with water, the larvae escape from the Cyclops into the human intestine. The disease caused by this worm is called dracunculiasis.
Trichuris__ The Whip worm:
Trichuris trichiura is whip shaped hence it is so named. It lives in the caecum, appendix and colon of man, especially children.
Loa-loa__ The Eye worm:
Loa loa lives in the sub-dermal connective tissue of man and baboons (large monkeys of Africa and Southern Asia with a dog-like face). Man gets infec­tion by bites of infected deerfly chrysops. Loa loa causes loiasis, a disease characterised by subcutaneous swelling (called calabar swelling) mostly around the eyes. This parasite may produce conjunctivitis in the eye.
These animals were originally grouped together because all seemed to possess a peculiar type of body cavity called a pseudocoel (that is, a body cavity that does not contain a lining of mesoderm), which develops differently from the body cavities of other animals. It has become clear, however, that these animals do not have close evolutionary linkages with one another, and each group has been placed in its own phylum. On the other hand, rotifers appear to be strongly allied with acanthocephalans, and eventually these two groups may be classified together in the same phylum. On the whole, the other former aschelminths may be closely related to arthropods because all exhibit molting at some point during development.

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