EVERYDAY SCIENCE

Detailed explanation-1: -To respond to stimuli, cnidarians use a rudimentary muscular system consisting of muscle cells lying in bands up and down the body wall and in a circle around the mouth cavity (Fig. 3.27). The body shortens when the vertical bands contract. If muscles on only one side contract, the body bends in that direction.

Detailed explanation-2: -The nerve net is the simplest form of a nervous system found in multicellular organisms. Unlike central nervous systems, where neurons are typically grouped together, neurons found in nerve nets are spread apart. This nervous system allows cnidarians to respond to physical contact.

Detailed explanation-3: -Cnidarians are considered “nerve net animals” even though their nervous systems include various forms of condensation and centralization. Yet, their broad, two-dimensional muscle sheets are innervated by diffuse nerve nets.

Detailed explanation-4: -A diffuse nerve net is a basic design of cnidarian nerve systems. Nerve net of the sea anemone Nematostella vectensis revealed by expression of a mCherry protein under nerve-specific elav promotor. Modified from [98].