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Phylum Annelida

Last Updated : 30 Nov, 2022
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Animals have definite shapes and sizes. They are unbranched except for the sponges. Animals’ organs are generally internal, e.g., liver, heart, kidney, lungs, brain, stomach, etc. The growth of an animal’s body is limited, and it stops long before death. The growth regions are not localized. They can move bodily from one place to another. They lack chlorophyll and are heterotrophic in their mode of nutrition. Animals have distinct excretory organs. Asexual reproduction occurs only in the lower animals. Animals are classified according to some basics like the number of germ layers present in the embryo; the symmetry of the body of the organism, mode of origin of mouth. The broad classification of Animalia is based on common fundamental features. The phyla of the animal kingdom are Porifera, Cnidaria, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata. Hereunder is a detailed explanation of the phylum Annelida.

Phylum Annelida 

Leech

 

  1. Annelids occur in freshwater, seawater, or moist soil. Some are free-living, some are burrowing and a few are parasitic.
  2. The body is segmented broadly divided into ring-like true segments.
  3. The body is soft extensile and triploblastic.
  4. Organ level body organization.
  5. The first animal to acquire true coelom.
  6. Coelomic fluid act as hydraulic skeleton.
  7. Both longitudinal and circular muscles are present.
  8. Straight and complete alimentary canal.
  9. Excretion with the help of nephridia.
  10. The first animal to have a close circulatory system.
  11. Blood is red due to hemoglobin dissolved in plasma.
  12. The mode of respiration is cutaneous respiration through moist skin.
  13. Locomotion organs are segmentally arranged paired lateral appendages, parapodia, chitinous setae, or chaetae.
  14. Sex may be unisexual (nereis) or maybe bisexual (Earthworm).
  15. Fertilization is external or internal.
  16. Development is direct or indirect, there is a free-swimming larvae stage(trochophore).
  17. The nervous system consists of a dorsal “brain” and a ventral nerve cord, having ganglia and lateral nerves in each body segment.

Classification of Annelida

On the basis of the number and presence or absence of setae, the phylum Annelida is divided into the following classes:

Polychaeta

  • They are mostly marine and have highly developed parapodia with numerous setae.
  • The development is indirect, undergoing metamorphosis with a free-swimming trochophore larva.
  • Examples: Nereis (Clam worm or sandworm or Ragworm), Aphrodite (Sea mouse), Chaetopterus (Paddle worm), Sabella (peacock worm), Arenicola (lugworm), Amphitrite, Terebella, Serpula(Fanworm).

Oligochaeta

  • They are semi-terrestrial or freshwater annelids.
  • They have few setae.
  • Clitellum is present.
  • There is no metamorphosis.
  • Example: Pheretima, Lumbricus, Tubifex.

Hirudinea

  • They are ectoparasitic annelids.
  • The clitellum is formed only during the breeding season.
  • Surrounding the alimentary canal is found botryoidal tissue.
  • Example: Hirudinaria, Pontobdella, Hirudo.

Archiannelida

  • External segmentation is absent.
  • Internal segmentation is present.
  • Parapodia and setae are absent.
  • Examples are Polygordius, and Dinophilus.

Echiurida

  • These annelids are without external and internal segmentation. 
  • Setae are rare. Usually, they have long prostomium.
  • Examples are Bonellia, and Echiurus.

Examples

Nereis

  • It is commonly called clam worm or sandworm or ragworm.
  • Nereis is unisexual, and its reproductive phase is called Heteronereis.
  • During development, a trochophore larva is present.
  • Each segment bears laterally one pair of fleshy projections, the parapodia, used in swimming.
  • They are cylindrical in shape, found not only in sandy areas, and they are adapted to burrowing.

Hirudinaria (Bloodsucking leech) 

  • It is a facultative ectoparasite of cattle.
  • It is sanguivorous.
  • Its saliva contains an anticoagulant called hirudin.
  • The body cavity is filled with a mesodermal botryoidal tissue formed of branching tubular cells 

Pheretima (Earthworm)

  • It is found in wet soil containing rich organic matter.
  • It is omnivorous, fossorial, nocturnal, hermaphrodite, and protandrous.
  • Furthermore, it possesses a great power of regeneration.
  • Morphology: Earthworm is brown or clay-colored due to the pigment porphyrin.
  • The body shows metamorphic segmentation. 
  • The digestive system consists of the alimentary canal and digestive glands.
  • The blood vascular system of earthworms is closed type.

Earthworm

Economic Importance of Earthworms

The earthworms are better known as the friend of farmers due to the following reasons:

  • Earthworms improve soil fertility in different ways; therefore, they are of utmost importance in agriculture.
  • These are used as bait and food. As bait is used in fishing. The earthworms were used as food by so many uncivilized people of the world and they are still used as food by the Macrea people.
  • Several people earn by catching the earthworms and supplying them to scientific laboratories.

FAQs on Phylum Annelida

Question 1: Write two special characteristics of annelids.

Answer: 

  • They respire through their body surface
  • Nephridia are excretory organs.

Question 2: What are the examples of annelids?

Answer:

Pheretima(Earthworm), Nereis, and Hirudinaria (Bloodsucking leech) are some of the examples of annelids.

Question 3: Define the term Setae.

Answer:

 A bristle or hair in many invertebrates. Setae are produced by the epidermis.

Question 4: Why earthworms are known as the friend of farmers?

Answer: 

Because the earthworms improve the fertility of the soil in different ways and, therefore, they are of utmost importance in agriculture.

Question 5: Name the first animal to acquire true coelom.

Answer:

Annelids are the first animal to acquire true coelom.


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