Phylum-Annelida: Definition, General Characters, Identifying Characters, Examples

The phylum-Annelida is a group of organisms having ring-shaped segments.
The term Annelida is derived from the Latin word

              ” Annulus = ring “

There are more than 15000 known species of this phylum. The phylum Annelida includes several subgroups based on their identities and anatomical variations. In 1802, Lamarck has first coined the term Annelida to the group of organisms belonging to the taxon vermis. Linnaeus was introduced the taxon vermis for the group of worms in 1893.

Definition:

Phylum Annelida are the group of organisms having bilaterally symmetrical triploblastic metamerically segmented ring-like body containing true coelom, ventral nerve cord, and thin external cuticle.

Fig: Hirudinaria sp.

General characters:

Some common characteristics features of the phylum Annelida are
1. The body is long cylindrical segmented ring-like.
2. Bilaterally symmetrical triploblastic body.
3. They have true coelom in their body.
4. The body is covered by thin cuticle layers.
5. Presence of a tubular and complete alimentary canal extending from mouth to anus.
6. In their body, the closed blood circulatory system is present.

Fig: Nereis sp.

7. The hemoglobin is present in the plasma, so the blood is red.
8. Presence of segmentally arranged setae as the locomotory organ.
9. Each segment of their body contains a pair of excretory organs called nephridia.
10. They have well-developed sense organs, and the nervous system is quite an advanced type.
11. Respiration occurs through the body wall.
12. Clitellum is present in all animals except the class Polychaeta.
13. Sexes may be separate, and some animals are hermaphrodites.
14. Trochophore larva is present in their life cycle.

Identifying characters Phylum Annelida:

1. Body structure: The body is cylindrical and segmented into rings-like.
2. The coelom: In their body, the coelom is present.
3. Locomotory organ: The Setae is present as an excretory organ.
4. Excretory organ: The excretory organ nephridia is present in each part of the body.
5. Life cycle: In their life cycle, trochophore larva is present.

Examples:

Some common animals of the Phylum Annelida are Pheretima(Earthworm), Nereis, Hirudinaria, Chaetopterus, Aphrodite, Serpula, Nerilla, Polygordius, Arenicola, etc.

Fig: Pheretima sp.

Questions and Answers:

1. Heart to pump blood evolved for the first time
a. Roundworms
b. Flatworms
c. Annelids
d. Arthropods

Answer:
c. Annelids

2. Annelida larva is
a. Trochophore
b. Maggot
c. Tadpole
d. Bipinnaria

Answer:
a. Trochophore

3. Clitellum is absent in
a. Eutypheus
b. Nereis
c. Pheretima
d. Hirudinaria

Answer:
b. Nereis

4. Which of the following is not a characteristic feature of the phylum Annelida
a. Segmentation
b. Hydrostatic skeleton
c. Pseudocoelom
d. Close circulatory system

Answer:
c. Pseudocoelom

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