Classification of Animal Kingdom: Annelida, Arthropoda and Mollusca

Our exploration of the animal kingdom progresses by examining additional phyla. Annelida, segmented worms, exhibit a distinct body organization with specialized excretory organs. In contrast, Arthropoda, the largest animal phylum encompassing insects, boast a hard chitinous exoskeleton and diverse respiratory adaptations.

Phylum- Annelida

About: They may be aquatic (marine and freshwater) or terrestrial, free-living, and sometimes parasitic.

• Features: Their body surface is distinctly marked out into segments or metameres and, hence, the phylum name Annelida.
  • Their excretory and osmoregulatory organ is called Nephridia.
• Example: Pheretima (Earthworm) and Hirudinaria (Bloodsucking leech).

Phylum- Arthropoda

About: This is the largest phylum of Kingdom Animalia, which includes insects. Over two-thirds of all named species on Earth are arthropods.

• Features: The body of arthropods is covered by a chitinous exoskeleton.
  • Respiratory organs are gills, book gills, book lungs or tracheal system.
  • The circulatory system is of open type.
  • Sensory organs like antennae, eyes (compound and simple), statocysts or balancing organs are present.
  • Excretion takes place through malpighian tubules.

• Examples: Economically important insects– Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect), Vectors– Anopheles, Culex and Aedes (Mosquitoes), Gregarious pest– Locusta (Locust), Living fossil– Limulus (King crab).

Phylum- Mollusca

About: This is the second largest animal phylum.

• Features: Molluscs are terrestrial or aquatic (marine or freshwater), having an organ-system level of organisation.
  • Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump.
• Example: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish).

Conclusion

These phyla highlight diversity within the animal kingdom. Annelida’s segmented bodies contrast with the chitinous armor of Arthropoda and the soft-bodied shells of Mollusca. Each phylum reflects a unique evolutionary solution, enriching diversity of animal form and function.

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