Examkin

Question

Difficulty: Very hardEvolutionary Trends and Adaptive Features in Organisms

Arrange the following animal organisms in order of increasing evolutionary complexity and cephalization of their nervous systems, starting from the most primitive structural arrangement to the most advanced.

  1. 1Hydra (Cnidaria), possessing a diffuse, uncentralized nerve net
  2. 2Planaria (Platyhelminthes), featuring paired anterior cerebral ganglia with a ladder-like nerve cord
  3. 3Earthworm (Annelida), exhibiting a bilobed cerebral ganglion connected to a solid ventral nerve cord with segmental ganglia
  4. 4Frog (Amphibia), possessing an advanced brain continuous with a dorsal hollow nerve cord encased in a protective column

Answer

The correct sequence from primitive to advanced nervous system organization is: Hydra (diffuse nerve net) → Planaria (anterior ganglia with ladder-like cord) → Earthworm (ventral cord with segmental ganglia) → Frog (dorsal hollow nerve cord with centralized brain).
The evolutionary trend of animal nervous systems progresses from diffuse, non-centralized networks to highly centralized dorsal control systems. Cnidarians like Hydra possess only an uncentralized nerve net. Platyhelminthes like Planaria introduced bilateral symmetry and initial cephalization via paired cerebral ganglia. Annelids like Earthworms developed a solid ventral nerve cord with segmental ganglia. Chordates like Frogs represent the most advanced stage with a dorsal hollow nerve cord and specialized brain.

Step-by-Step Solution

1
Identify the most primitive tissue-level organism lacking nervous centralization.
Hydra (Cnidaria) has no brain or ganglia, operating solely on an interconnected network of nerve cells (nerve net).
Radial symmetry in lower invertebrates correlates with non-directional diffuse nerve nets.
2
Determine the onset of bilateral symmetry and primitive cephalization.
Planaria (Platyhelminthes) introduces paired cerebral ganglia at the anterior head end linked to transverse nerve cords.
Bilateral movement promoted head-first exploration, driving concentration of sensory structures at the anterior end.
3
Identify coelomate invertebrate centralization with metameric segmentation.
Earthworms (Annelida) feature a ventral nerve cord with prominent ganglia repeating in each body segment.
Segmented coelomates evolved localized motor control per segment coordinated by a central ventral trunk.
4
Select the chordate displaying maximum cephalization and dorsal protection.
Frogs (Amphibia) possess a tripartite brain and a dorsal hollow spinal cord protected by vertebrae.
Vertebrate evolution shifted nerve cord position dorsally and concentrated complex processing centers in a skull.

Key Concept

Evolutionary Trends in Neurological Organization and Cephalization
Rate this question

Topics

Evolutionary Trends and Adaptive Features in Organisms

More questions from this topic