Annelida: Metamerism | Zoology Optional Notes for UPSC PDF Download

Metamerism in Annelida: A Detailed Exploration

• Metamerism is first observed in the animal kingdom within the phylum Annelida.
• Annelids exhibit a highly developed form of metamerism, where each segment is constructed based on a fundamental plan and contains essential components of various body systems.
• Metamerism is also found in kinorhynchs, arthropods, and many chordates.

Characteristic Features of Metamerism

• Segmental Distribution: Metamerism is typically confined to intermediate (trunk) segments, with the head (acron) and posterior pygidium or telson remaining non-segmented.
• Mirror Image: Each metamere is a mirror image of the others, maintaining symmetry throughout the body.
• Interdependence: Segmental structures are interdependent, forming a cohesive functional unit.
• Integrated Functionality: All segments of the body work in coordination to ensure the animal's overall functionality.

Types of Metamerism

1. True Metamerism: Involves the segmentation of the body through the segmentation of the mesoderm. Annelids, arthropods, and many chordates exhibit this type.
2. Homonomous Metamerism: Refers to a type where all segments or somites in an animal are identical. Annelids demonstrate homonomous metamerism.
3. Heteronomous Metamerism: Arthropods and chordates exhibit heteronomous metamerism. Segments in different body regions are dissimilar and restricted to specific organs.
4. External Metamerism: Manifests externally in arthropods, with no internal partitions marking the segments.
5. Internal Metamerism: Vertebrates display internal metamerism, predominantly in embryonic stages, affecting the muscular, skeletal, and nervous systems.
6. External and Internal Metamerism: Annelids exhibit both external and internal metamerism. Body segments are externally marked by skin constrictions and internally by partitions (septa).
7. Complete Metamerism: Evident in annelids, as segmentation is practically observed in all body systems.
8. Incomplete Metamerism: Arthropods and chordates exhibit incomplete metamerism, where not all organs display segmentation.
9. Pseudo-metamerism or Strobilization: Seen in tapeworms (Platyhelminthes) and differs significantly from true metamerism.

Theories on the Origin of Metamerism: Unraveling the Evolutionary Enigma

Pseudo-metamerism Theory

• This theory suggests that metamerism evolved as a secondary development, resulting from the repetition of body parts in ancestors that were originally acoelomate and un-segmented.
• Turbellarians exhibited this pseudo-segmentation, which eventually gave rise to metamerism.

Cyclomerism Theory

• Proposed by Sedgwick in 1884, this theory aligns with the enterocoelous theory of coelom origin.
• According to this theory, the coelom originated from the gastric pouches of ancestral anthozoan coelenterates, which later transformed into coelomic pouches in protocoelomates.
• In annelids, further subdivision of the metacoel led to the emergence of segmented individuals.

Corn or Fission Theory

• The Corn or Fission Theory proposes that metamerism resulted from incomplete separation following repeated transverse divisions of a non-segmented ancestor.
• It also accounts for asexual reproduction leading to the formation of a chain of sub-individuals or zooids that are joined end to end.

Locomotion Theory

1. Introduced by R. B. Clark (1964), this theory explains the origin of metamerism in annelids as an adaptation for peristaltic locomotion and burrowing.
2. In chordates, metamerism independently evolved to enable the undulatory movement of the body's tail muscles.

Significance of Metamerism

• Enhanced Locomotion: Metamerism plays a vital role in various forms of locomotion, including burrowing, by allowing the controlled contraction and relaxation of body segments.
• Division of Labor: Metamerism permits division of labor among different segments of the body, optimizing various physiological functions.

These theories provide valuable insights into the origins and implications of metamerism, a remarkable structural phenomenon observed in various organisms across the animal kingdom.