How do you tell the difference between land hermit crabs and marine?

i just found 5 hermit crabs on the beach, so mom said i could take em home, after researching them. little did we know that there where two types of hermit crabs! i need to know the dif, to know how to take care of them, and set up their cage. right now the cage is hopefully suiting either type. it seems that they are staying in the water a lot but also some are on the land, sooo i need to know how to tell the dif. please help! thanks