Concept: This question tests your knowledge of the evolution and types of nervous systems in different animal phyla, specifically focusing on the most primitive forms.

Why (A) is correct: Hydra, belonging to the phylum Cnidaria, exhibits a diffuse nervous system or nerve net. In this type, nerve cells are scattered throughout the body wall, forming a network without a centralized brain or ganglia. This allows for a generalized response to stimuli.

Why other options are wrong:

• (B) Planaria: Planaria (Platyhelminthes) have a more organized ladder-like nervous system with a primitive brain (ganglia) and longitudinal nerve cords.
• (C) Sponges: Sponges (Porifera) are the simplest multicellular animals and lack a true nervous system altogether.
• (D) Vertebrates: Vertebrates possess a highly complex and centralized nervous system with a well-developed brain and spinal cord.

Common Mistake: Students might confuse the absence of a nervous system in sponges with the diffuse nervous system of cnidarians.

NEET Tip / Mnemonic: Remember the evolutionary progression: Sponges (no nervous system) → Cnidarians (diffuse nerve net) → Platyhelminthes (ladder-like) → Annelids/Arthropods (ganglionic/ventral nerve cord) → Vertebrates (centralized brain and spinal cord).

Correct Answer: (A)