Sea anemones wait for their prey to come to them and it is the tentacles, swaying in the current, which sense that prey is nearby. They have a simple nervous system, to help with movement and senses.

Jellyfish have a neural network distributed throughout their body. This allows jellyfish to respond to stimuli from any part of their body. However, they really rely on chemical senses to detect good or bad in the water around them. They also have sense organs which repond to light and gravity.

Corals have a simple nervous system called a nerve net, which extends from the mouth to the tentacles. Polyps can detect substances such as sugars or amino acids. This sense is similar to out sense of taste and it allows the polyps to detect when prey is nearby. They also have tiny sensors in the tentacles which trigger the nematocyst to eject its sting. These sensors can be stimulated either physically or chemically.