Animals are those organisms which are eukaryotic, multicellular and heterotrophic in their mode of nutrition. Animal cells do not have cell walls. Except for a few, most animals are mobile. Multicellular animals are often called metazoa and have been divided into two branches: Parazoa and Eumetazoa. In Parazoa, the animal’s body is formed of loosely aggregated cells. These animals have poorly differentiated tissues and have no organ or digestive cavity (e.g., sponges). However, in Eumetazoa, the cells are properly arranged into tissues and organs. Their digestive track is the mouth but with or without an anus. 

Eumetazoa is further classified on the basis of 

• the number of germ layers present in the embryo
• the symmetry of the body of the organism
• the mode of origin of mouth

Those animals which have two germ layers in the embryo are called diploblastic animals and those having three germ layers are called triploblastic animals. Further classification of animals is on symmetry, on basis of mouth origin, based on the presence or absence of body cavity or coelom. Animals do not have cell walls and chlorophyll. The central vacuole is absent. They store food as glycogen and fat. Animals can transmit impulses due to the presence of nerve cells. Sponges do not have nerve cells. Centrioles are present. 

Phylum Aschelminthes 

 

Distinguishing Features

1. Many roundworms live as parasites in plants and animals. They cause serious agricultural, veterinary and human health problems. A large number of nematodes are free living and occur in freshwater, seawater and soil.
2. They are called roundworms because they appear circular in cross-section. They are unsegmented.
3. The body wall consists of a firm, non-living resistant cuticle, syncytial epidermis (a continuous layer of cytoplasm with scattered nuclei) and muscle layer. The muscle layer consists of longitudinal fibres only. The epidermis lacks cilia.
4. The body cavity is pseudocoel or pseudocoelom.
5. Digestive Tract is complete with an anterior mouth and posterior anus with the muscular pharynx. The intestine is non-muscular.
6. Skeletal, respiratory and circulatory systems are absent. Pseudocoelomic fluid present in the pseudocoelom maintains body shape. It is called hydroskeleton. Gaseous exchange in aerobic respiration occurs by diffusion through the body surface. The pseudocoelomic fluid transports materials.
7. Ammonia is the main excretory matter. Ascaris excretes both ammonia and urea.
8. The nervous system consists of a circumpharyngeal ring that gives rise to nerves, forwards as well as backwards.
9. Papillae and amphids are present on the lip and phasmids on the posterior end of the body.
10. The roundworms are usually dioecious(unisexual). Generally, they show sexual dimorphism. 
11. Fertilization is internal. There is no asexual reproduction.

Classification of Aschelminthes

Aschelminthes is classified into 2 classes i.e., Aphasmidia and Phasmidia.

Aphasmidia

• Phasmids are absent.
• Excretory organs are reduced or absent.
• Examples:- Trichinella, Trichuris, etc.

Phasmidia

•  Phasmids are present.
•  Excretory organs are well developed. 
•  Examples:- Ascaris, Enterobius, Ancylostoma, Wuchereria, etc.

Examples of Achelminthes

Ascaris

Ascaris lumbricoides is an endoparasite of the small intestine of human beings. It is more common in children. Ascaris shows sexual dimorphism. Male is smaller than female. The posterior end of the male is curved. A cloaca and two equal spicules projecting through the cloacal aperture are present in male Ascaris. Two equal spicules project through the cloacal aperture. Female Ascaris has a separate anus and genital aperture. Spicules are absent in female Ascaris. Adult Ascaris lives for about 9 to 12 months in the host.

Ascaris is both ammonotelic and ureotolic. If free oxygen is available in traces, Ascaris is a facultative anaerobe.

Amphids are two pits upon ventro- lateral lips of the mouth. These are chemoreceptors. Phasmids are present on lateral sides of the body in the posterior region and are glandulosensory in nature.

The posterior end of the male bears 50 pairs of preanal papillae in front of the cloacal aperture and 5 pairs of postanal papillae. These papillae of the male help in copulation.

The whole excretory system of canals is formed of a single highly elongated and complicated giant cell. This cell is called Renette cell. Its nucleus is located upon the left longitudinal canal close to the connecting bridge. Three other smaller nuclei are also found, one upon connecting bridge and two upon terminal duct.

Ancylostoma – The hookworm

It is found as an endoparasite in the small intestine of human beings.

Ancylostoma duodenal(Hookworm) causes acute anaemia. It also retards the growth of the body in children. The disease is called ancylostomiasis. Its infection is more prevalent in rural areas where people move bare-footed and hygiene is neglected.

Wuchereria (Filaria) – The Filaria 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 Cupex and Aedes mosquitoes) is the intermediate host. Thus its lifecycle is digenetic.

Filaria is viviparous. In the lymph glands, the juveniles develop into adults. The accumulation of these worms blocks the lymphatic system resulting inflammation of lymph gland and other pathological conditions. In some cases, the obstruction of lymph vessels may cause a serious disease known as elephantiasis(=Filariasis) in the legs and arms.

Dracunculus – The Gunlea worm

It 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 freshwater crustacean, which is the intermediate host of the guinea worm. When the infected cyclops is taken in by man with water, the larvae escape where they become adults. The guinea worm causes itching, diarrhoea, nausea, vomiting and eosinophilia due to its toxic secretions in the human body.

Diseases caused by Aschelminthes

Aschelminthes are known to cause various diseases in plants, animals and also humans. Some of the diseases are:

1. Ascariasis: The causal agent of this disease is Ascaris lumbricoides in humans. It is very common in children. Symptoms of Ascariasis are abdominal pain, diarrhoea, and vomiting. This disease is a kind of roundworm infection.
2. Ancylostomiasis: It is caused due to hookworm infection in the small intestine. It causes anaemia. Worms house the small intestine in a large number and get stuck to its intestinal wall and cause anaemia by sucking blood.
3. Filariasis: It is caused by roundworms.  lymphatic filariasis is caused by Wuchereria bancrofti, Brugia timori and Brugia malayi. It causes thickening of the skin and tissue present beneath the skin.
4. Enterobiasis: Enterobiasis is caused by pinworm Enterobiasis vermicularis. Itching in the anal area and sleeping discomfort are caused by this disease.

FAQs on Aschelminthes

Question 1: Write the sense organs of phylum Aschelminthes.

Answer:

Papillae, Amphids and Phasmids are the sense organs of phylum Aschelminthes.

Question 2: Which is the causal organism for the disease ascariasis? 

Answer: 

Ascaris is an endoparasite which causes asacariasis.

Question 3: What are the symptoms of ascariasis?

Answer:

Fever, abdominal swelling, abdominal discomfort, and abdominal cramping are the symptoms of ascariasis.

Question 4: Give examples of Aschelminthes.

Answer:

Ascaris, Wuchereria, and Dracunculus are some of the examples of phylum aschelminthes.

Question 5: Write the difference between male Ascaris and female Ascaris.

Answer:

•  Males are smaller than females. 
• The posterior end of the male is curved while that of the female is straight.