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Phylum Aschelminthes

Introduction:

  • Phylum Aschelminthes is also known as phylum Nemathelminthes.
  • The phylum includes Roundworms i.e., worms with cylindrical body, as their body is circular from the cross-section.
  • There are about 15,000 species belonging to phylum Aschelminthes.

Topics covered

  • General characteristics of phylum Aschelminthes
  • Classification of phylum Aschelminthes
  • Examples
  • Life cycle of Ascaris lumbricoides
  • Life cycle of Wuchereria bancrofti

General characteristics of phylum Aschelminthes -
 

Detailed explanation:

  • Habit and habitat
     
  • Most of them are parasitic worms in plants and animals
  • Some are free living.
  • Both aquatic and terrestrial.
     
  • Level of Organisation
     
  • Organ system level of Organisation.
     
  • Symmetry
     
  • Bilaterally symmetrical.
     
  • Germ layers
     
  • Triploblastic animals.
     
  • Body wall
     
  • Body wall is made up of layers-
  • Cuticle - Outer, firm,water impermeable and non living.It acts as a protective cover against digestive juices,in endoparasites.
  • Syncytial epidermis- Continuous layer that contains cytoplasm with scattered nuclei.
  • Muscle layer - Longitudinal muscle fibres are present.
     
  • Body plan
     
  • Tube-within-tube body plan.
     
  • Coelom
     
  • Pseudocoelomates i.e., mesoderm is present in the form of scattered pouches between the ectoderm and endoderm.
     
  • Digestion
     
  • Complete digestive system is present.
  • Alimentary canal includes mouth, muscular pharynx, intestine and anus.
     
  • Respiration
     
  • Respiration takes place by the general body surface through diffusion.
  • Anaerobic respiration present in endoparasites.
     
  • Excretion
     
  • Excretion takes place through H-shaped Renette cells that consist of excretory canals or tubes (protonephridia).
  • Excretory canals or tubes remove body wastes from the body cavity through the excretory pore.
  • These are ammonotelic i.e ammonia is the nitrogenous waste.
  • Ascaris excretes urea.
     
  • Skeleton
     
  • Pseudocoelomic fluid is present in the pseudocoelom that maintains the body shape. It forms the hydroskeleton.
     
  • Nervous system
     
  • Nervous system consists of the circumpharyngeal rings and forms the anterior as well as posterior nerves.
  • Sense organs are found in them -
  • For eg,
  • Papillae (tactile or tangoreceptors - respond to touch) are found on the lips.
  • Amphids (chemoreceptors - responds to chemicals) are found on lips.
  • Phasmids (chemoreceptors) are found on the tail.
     
  • Reproduction
     
  • Reproductive system is well-developed.
  • These are dioecious i.e sexes are separate or Male and female are distinct.
  • Show sexual dimorphism i.e., male and female can be distinguished on the basis of morphology.
  • Often females are longer than males.
  • Development may be direct (young one resembles the adult) or indirect (larval stages are formed).
  • For eg,
  • Larvae of Ancylostoma (hookworm) - Filariform
  • Larva of Wuchereria (filarial worm) - Microfilaria
  • Larvae of Ascaris and Enterobius (pinworm) - Rhabditiform
  • Differences between male and female Roundworms -
     
Male Female
They are smaller than females. They are larger than male
Posterior end of the tail is curved. Posterior end of the tail is straight.


Male Female
Cloaca is present which opens outside through a cloacal aperture Cloaca is absent. Separate anus and genital aperture is present



ascaris

Classification of phylum Aschelminthes
 

Detailed explanation:

Phylum Aschelminthes is divided into two classes

1. Aphasmida
 

  • Phasmids are absent.
  • Various types of amphids are present.
  • Example - Trichinella (Trichina worm) etc.

2. Phasmida
 

  • Phasmids are present in them.
  • Amphids are present near the anterior end.
  • Examples - Ascaris lumbricoids (Intestinal round worm), Enterobius (Pin worm), Ancylostoma (Hookworm), Wuchereria (Filarial worm)etc.

Examples
 

Detailed explanation:

1. Ascaris

  • They are commonly known as “The Intestinal Roundworm”.
  • These are endoparasites (live inside the body of the host) of the small intestine of human beings, especially in children.
  • Adults can live for about one to two years.
  • The mouth of Ascaris is bounded by three lips i.e. one dorsal and two ventrolateral. the dorsal lip.

2. Ancylostoma duodenale -

  • Commonly known as “Hookworm”.
  • These are the endoparasites in the intestine of human beings. The larvae enter the body of the human from moist soil by boring through the skin of the feet.
  • The intermediate host is not required.
  • It causes a disease known as ancylostomiasis.

3. Wuchereria -

  • Commonly known as “ filarial worm”.
  • These are viviparous.
  • These are endoparasites.
  • Adults live in the lymphatic vessels and lymph nodes of human beings.

    wuchereria
     

4. Enterobius

  • Commonly known as “Pin worm”.
  • These are endoparasites and found in the caecum, colon or vermiform appendix of human beings.

5. Trichinella

  • Commonly known as “ Trichina worm”.
  • They cause a disease known as trichinellosis.

6. Dracunculus

  • Commonly known as “Guinea worm”.
  • They cause a disease known as dracunculiasis.

7. Trichuris trichuris

  • Commonly known as “Whip worm”.

8. Loa-loa

  • They are commonly known as “Eye worm”.
  • This parasite may produce conjunctivitis in the eye.

Life cycle of Ascaris lumbricoides
 

Detailed explanation:

  • They cause a disease known as ascariasis.
  • The symptoms of ascariasis are -
    - Fever
    - Abdominal discomfort
    - Abdominal swelling (in children)
    - Abdominal cramping
  • They are monogenetic i.e require only one host (Man)

    life-cycle-ascaris
     

Life cycle of Wuchereria bancrofti -
 

Detailed explanation:

  • They cause a disease in humans known as elephantiasis/filariasis.
  • Symptoms of this disease include
    - Internal bleeding
    - Muscular pain
    - Fever
    - Anaemia
    - Blockage of the intestinal passage
    - Oedema of lower limbs, due to lymphatic blockage.
  • They are digenetic parasites.
    - Primary host - Man
    - Secondary host - Certain species of Culex mosquito.
  • Their life cycle includes following steps -
    - An infected mosquito introduces the third-stage of the larvae into the skin of the human host, during the blood meal.
    - They develop in adults which are commonly found in the lymph nodes.
    - Adults produce the microfilariae (first stage larvae/L1 larvae)which migrate into lymph and blood channels through lymph and blood.
    - Now, certain species of culex mosquito ingests the microfilariae during a blood meal.
    - After ingestion, the microfilariae lose their sheaths and reach the thoracic muscles of the mosquito.
    - There the microfilariae develop into second stage larvae and then further into third-stage infective larvae
    - The third-stage larva is infective which migrate through the hemocoel to the mosquito’s proboscis and can infect another human when the mosquito takes a blood meal.

    wuchereria-bancrofti
     

Frequently Asked Questions: FAQs
 

Q1. Write the name of the excretory structure of phylum Aschelminthes.
Ans:

  • Excretion takes place through H-shaped Renette cells that consist of excretory canals or tubes (protonephridia).
  • An excretory canals or tubes removes body wastes from the body cavity through the excretory pore.

Q2. Write the sense organs of phylum Aschelminthes.
Ans:

  • Sense organs found in them are following -
    - Papillae (tangoreceptors - responds to touch) are found on the lips and are tactile in function.
    - Amphids (chemoreceptors - responds to chemicals) are found on lips.
    - Phasmids (chemoreceptors) are found on the tail.

Q3. Write the difference between the male Ascaris and female Ascaris.
Ans:

 

Male Female
They are smaller than females. They are larger than male
Posterior end of the tail is curved. Posterior end of the tail is straight.
Cloaca is present which opens outside through a cloacal aperture Cloaca is absent. Separate anus and genital aperture is present


Q4. Which is the causal organism for the disease ascariasis ?
Ans:

  • The disease ascariasis is caused by Ascaris which is an endoparasite and their adult lives in the lymphatic vessels and lymph nodes of human beings.

Q5. What are the symptoms of ascariasis?
Ans:

  • The symptoms of the disease ascariasis are -
    - Fever
    - Abdominal discomfort
    - Abdominal swelling (in children)
    - Abdominal cramping

Q6. Why are Aschelminthes commonly called Roundworms ?
Ans:

  • The phylum Aschelminthes is commonly called Roundworms because their body is circular from the cross-section.

Q7. Which is the causal organism for the disease elephantiasis.
Ans:

  • The disease elephantiasis is caused by Wuchereria (The filarial worm) which are endoparasites as their adult lives in the lymphatic vessels and lymph nodes of human beings.

Q8. What are the symptoms of elephantiasis?
Ans:

  • Symptoms of the disease elephantiasis include -
    - Internal bleeding
    - Muscular pain
    - Fever
    - Anaemia
    - Blockage of the intestinal passage
    - Oedema of lower limbs, due to lymphatic blockage.

Q9. What is the site of first moulting in ascaris ?
Ans:

  • In egg present in moist soil

Q10. what is the site of second and third moulting in ascaris
Ans:

  • Second and third moulting occurs in the alveoli of infected human.

Q11. Where does fourth moulting in ascaris occurs ?
Ans:

  • Fourth moulting occurs in the small intestine of infected humans and it converts L4 larvae into adult worms.
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