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Difficulty: MediumEvolutionary Trends in Plants and Animal Systems

Arrange the following structural body plans of animals in order of increasing evolutionary complexity, from the most primitive organization to the most advanced.

  1. 1Diploblastic body plan with two germ layers and radial symmetry
  2. 2Triploblastic acoelomate body plan with bilateral symmetry but no internal body cavity
  3. 3Triploblastic pseudocoelomate body plan with a body cavity partially lined by mesoderm
  4. 4Triploblastic coelomate body plan with a true body cavity completely lined by mesodermal epithelium

Answer

The correct evolutionary sequence of animal body plans from primitive to advanced is: Diploblastic body plan with two germ layers and radial symmetry → Triploblastic acoelomate body plan with bilateral symmetry but no internal body cavity → Triploblastic pseudocoelomate body plan with a body cavity partially lined by mesoderm → Triploblastic coelomate body plan with a true body cavity completely lined by mesodermal epithelium.
The correct order reflects the fundamental evolutionary sequence of animal organization: primitive radial diploblastic forms evolved first, followed by bilateral triploblastic acoelomates, then pseudocoelomates with a persistent blastocoel, and finally coelomates possessing a peritoneum-lined true coelom.

Step-by-Step Solution

1
Identify the germ layer organization and symmetry of primitive multicellular animals.
Diploblastic animals possessing two germ layers (ectoderm and endoderm) with radial symmetry evolved first among tissue-grade organisms.
Radial diploblastic body plans preceded the development of mesoderm and bilateral symmetry.
2
Determine the emergence of the third germ layer (mesoderm) and bilateral symmetry without a body cavity.
Triploblastic acoelomate organization evolved next, introducing a middle germ layer (mesoderm) and cephalization, but remaining solid between digestive tract and body wall.
Acoelomates represent the first triploblastic lineage prior to body cavity formation.
3
Distinguish between false body cavity (pseudocoelom) and true body cavity (coelom) evolution.
Pseudocoelomates evolved a cavity derived from the blastocoel, followed by coelomates where mesoderm fully lines the cavity to form a peritoneum.
A true coelom allows complex organ suspensions and muscularized digestive tracts.

Key Concept

Evolutionary progression of germ layers, body symmetry, and body cavity organization in animals
Estimated Time:1m 0s
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Topics

Evolutionary Trends in Plants and Animal Systems

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