A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate.
The first step in sensation is reception. It makes use of accessory structures or sensory receptors which modify or respond to incoming stimuli. A stimuli is defined as a detectable change inside or outside an environment. Reception is the activation of these accessory structures by a stimuli. Examples of a stimuli are changes in temperature, pain, light, loud sound, etc.