On the limpet deal, here is the answer I provided.  Right or wrong, it is a
pretty good theory.

Tom Eichhorst in New Mexico, USA

"Tell your young friend that it may be due to the need for cooling on the
dry
rocks.  According to Vermeij (A Natural History of Shells, 1993, pp. 66) the
shell shape of limpets and nerites seems to vary with their place in the
intertidal zone.  The tall, high conical shape of the limpets in the high
intertidal zone is (like their neighboring nerites) a function of the need
for cooling.  With this shell shape they are able to retain more water
within the shell and use this water to slowly cool themselves during the
period they are exposed to the sun by an evaporative process around the base
of the shell.  The shells found lower down are flatter in shape as they do
not need to retain any extra water and the flatter shape helps them with
stand the pressures of the waves.  The Littorine species (periwinkles) do
not have this need for cooling as they basically shut down almost all life
processes when they are high and dry.  Like a short term hibernation, this
allows them to withstand the drying out and heat without a need for
additional cooling -- thus their shell shape does not vary."