Hermit crabs range from these little guys, scavenging the back end of rocks along the shore
to the landlubbers known as “soldier crabs,” which can get up to baseball sized, shell (usually the West Indian topshell or whelk as below) included.
These are the ones who swarm out of the mountains in August to mate by the sea.
There is another land crab, not a hermit, which I only caught glimpses of. 
Mud around their nest holes is evidence of recent activity below.
They are still hunted for food so their shyness is excusable.
Backyard and Beyond 
I’m fond of hermit crabs, although I’ve never seen one the size of a baseball. Just the little guys in tidal pools. But baseball-sized soldier crabs swarming down out of the hills? Egad. Actually it all sounds quite fascinating.