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Difficulty: Very hardHigher Invertebrates: Annelida, Mollusca, Arthropoda, and Echinodermata

Consider the structural evolution of body segmentation (metamerism) and appendage specialization among higher invertebrates. Arrange the following organisms in ascending order of structural complexity, starting from the simplest homonomous metamerism to the highest degree of tagmatization and organ specialization. Which sequence represents the correct order?

  1. 1Earthworm (Phylum Annelida)
  2. 2Centipede (Phylum Arthropoda, Class Chilopoda)
  3. 3Crayfish (Phylum Arthropoda, Class Crustacea)
  4. 4Grasshopper (Phylum Arthropoda, Class Insecta)

Answer

The correct order from simplest segmentation to most specialized tagmatization is Earthworm (Phylum Annelida) → Centipede (Class Chilopoda) → Crayfish (Class Crustacea) → Grasshopper (Class Insecta).
The sequence follows the evolutionary transition of body segmentation: starting with the earthworm (homonomous metamerism without jointed appendages), moving to the centipede (head and long trunk with jointed legs), advancing to the crayfish (two tagmata: cephalothorax and abdomen), and culminating in the grasshopper (three distinct, highly specialized tagmata: head, thorax, and abdomen).

Step-by-Step Solution

1
Identify the body organization of Annelida (Earthworm).
Annelids exhibit homonomous metamerism where body segments are nearly identical, with simple unjointed chaetae and no specialization into tagmata.
This represents the ancestral coelomate segmented body plan.
2
Evaluate tagmatization in Myriapoda (Centipede).
Centipedes show primitive arthropod tagmatization into a distinct head and a long trunk with repetitive jointed appendages.
Jointed limbs evolve, but trunk segments remain unspecialized.
3
Assess segment fusion in Crustacea (Crayfish).
Crustaceans fuse head and thoracic segments into a single cephalothorax covered by a carapace, leaving a distinct abdomen.
Tagmatization progresses to two distinct functional units with biramous appendages.
4
Determine maximum tagmata specialization in Insecta (Grasshopper).
Insects possess three clearly defined tagmata: head (sensory/feeding), thorax (locomotion with legs and wings), and abdomen (visceral functions).
Insect morphology represents the highest evolutionary specialization of metameric tagmatization.

Key Concept

Tagmatization and Body Plan Specialization in Higher Invertebrates
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Topics

Higher Invertebrates: Annelida, Mollusca, Arthropoda, and Echinodermata

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