Both creatures share the basic mollusc features of a soft body, comprising a muscular foot and a mass of various digestive and reproductive tissues on top of that, all covered by a hard shell. The shell of a mollusc isn't a suit of armour like that worn by crust-aceans such as the crab. Molluscs, unlike crustaceans, never have to cast off their shell or re-form it to provide growing room. This is a little like a rasping tongue that runs over a set of pulleys, but they use it for different purposes. The dog-whelk's radula drills into other animals such as barnacles, while the limpet's is used to rasp algae off rock surfaces.
Limpets seen under water while diving at high tide are, in fact, usually grazing.
Look out for these home bases, and the owners grazing nearby, next time you dive from a limpet-strewn shore. As we venture into slightly deeper water on our mollusc-spotting dive, we might well encounter a colourful nudibranch sea slug hungrily creeping over a rock covered with hydroids. Nudibranchs are closely related to limpet-like creatures, but seem different because they have no shell. Their gills are unprotected and appear as frilly rosettes or long finger-like projections on their backs. Nudibranch means ''naked gill''.