This is a leaf barnacle, most likely Pollicipes polymerus. Like most intertidal animals, they are subject to solar radiation, extreme salinity fluctuations, and heat (among other things), and have no way to regulate their body temperature internally. So how do these guys stay cool?
First, you get a good placement. The leaf barnacles live in the crevices of mussel beds [1,2]. The mussel beds, because of the closeness of the animals can retain a bit of cool water to help combat the heat. Living in shady cervices also helps. Additionally, most intertidal animals (especially barnacles) have special proteins that can help protect other proteins from unfolding and becoming useless in the heat. Some barnacles can function in 34 degrees C for several hours, without their proteins degrading (That's 93 degrees F for you Americans) . But leaf barnacles have a third way of keeping cool. They can lose up to 40% of their body water from their fleshy stalk . This evaporation process helps keep their body temperature slightly cooler than to ambient temperature.