Which of the following groups of animals do not possess a nervous system?

Analyze the question: You are asked which group of animals has no nervous system at all.

Explain the concept: A nervous system is a network of neurons (nerve cells) that detects stimuli and coordinates responses. Most animal phyla have one, but the simplest multicellular animals do not.

Go through the options:

• A. Porifera (sponges): These are the simplest multicellular animals. They have no true tissues, no organs, no neurons and no nervous system. They filter-feed using pores (ostia) and collar cells (choanocytes). Individual cells respond to stimuli on their own.
• B. Cnidaria (Hydra, jellyfish, sea anemone): They have a nerve net, a simple, diffuse nervous system. So they do have a nervous system.
• C. Platyhelminthes (flatworms such as tapeworm and planaria): They have a ladder-like nervous system with a simple brain (cerebral ganglia) and longitudinal nerve cords.
• D. Nematoda (roundworms): They have a nerve ring around the pharynx and longitudinal nerve cords.

Answer: Only Porifera lack neurons and a nervous system, so the correct option is A.

Tip: Remember the order of increasing complexity of nervous systems: Porifera (none) → Cnidaria (nerve net) → Platyhelminthes (ladder-type, simple brain) → higher animals (central nervous system). If an exam question asks about "no nervous system," think of sponges first.