Characteristics of Arthropoda: Study Notes

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General Characteristics of Arthropoda: The Jointed-Foot Animals

The phylum Arthropoda is the most diverse and abundant group in the animal kingdom, with a wide array of species adapted to nearly every habitat on Earth. Here are the primary characteristics that define arthropods.

1. Name Meaning

• The term Arthropoda is derived from Greek words where "arthro" means joint and "pod" means foot or leg, referring to their characteristic jointed appendages.

2. Phylum Size

• Arthropoda is the largest phylum in the kingdom Animalia, containing approximately 1 million known species and accounting for a vast diversity of life forms.

3. Animal Proportion

• Arthropods make up an impressive 83% of all known animal species, highlighting their overwhelming presence and diversity within the animal kingdom.

4. Body Organization

• Arthropods exhibit multicellular, triploblastic body structure with bilateral symmetry. They are coelomate (possessing a true body cavity) and protostomes (where the mouth forms before the anus during development).

5. Appendages

• Arthropods are characterized by their jointed appendages, which aid in locomotion, feeding, and other specialized functions.

6. Exoskeleton

• A hard chitinous exoskeleton covers the body, providing protection, structural support, and aiding in water retention. However, this exoskeleton also limits growth, necessitating periodic moulting (shedding).

7. Body Segmentation

• Arthropods exhibit metameric segmentation, with the body segmented externally and internally into functional regions.

8. Body Division

• The arthropod body is typically divided into three main regions:
  • Head: Contains sensory organs and mouthparts.
  • Thorax: Houses appendages like legs and wings (in some groups).
  • Abdomen: Contains organs for digestion, excretion, and reproduction.

9. Digestive System

• Arthropods have a complete alimentary canal, meaning a digestive system with a distinct mouth and anus, facilitating efficient digestion and nutrient absorption.

10. Respiration

• Respiratory organs vary across arthropod groups:
  • Gills: Common in aquatic species.
  • Book Lungs: Found in spiders.
  • Trachea: Network of tubes in insects that supplies oxygen directly to tissues.

11. Circulatory System

• Arthropods have an open circulatory system where blood, or haemolymph, flows freely within body cavities. Arthropod blood is colorless due to the lack of hemoglobin.

12. Body Cavity

• Their body cavity is referred to as a haemocoel, a large cavity where haemolymph circulates, making them haemocoelomate animals.

13. Excretion

• Excretory functions are carried out by specialized malpighian tubules, which filter waste from the haemolymph into the digestive tract for removal.

14. Vision

• Many arthropods possess compound eyes that provide mosaic vision, allowing for detailed and panoramic views of their surroundings, advantageous for detecting movement and predators.

15. Sensory Power

• Arthropods are known for having highly developed sensory organs, enabling them to detect environmental changes and respond with refined behavioral adaptations.

16. Special Features

• Metamorphosis: A significant adaptation in many arthropods, allowing distinct life stages (e.g., larva, pupa, adult) that minimize competition for resources.
• Moulting/Ecdysis: The process of shedding their exoskeleton to allow growth, followed by the hardening of a new, larger exoskeleton.

Arthropods, with their complex body structures, jointed appendages, and adaptive capabilities, dominate almost all ecosystems. Their remarkable evolutionary success is due in part to these unique characteristics, which enable them to thrive in both aquatic and terrestrial habitats, as well as a vast range of ecological niches.

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