the sea-fairies seldom swim , but wander singing over the bed of the sea in smooth places , or drive in shell-carriages harnessed to the tiniest fishes ; or else they ride astride little green crabs with bridles of fine threads ( which of course do n't prevent the crabs from going sideways , as they always will ) ; and they have troubles with the sea-goblins that are larger , and ugly and rowdy , and do nothing except fight and hunt fish and gallop about on sea-horses .