These little dorids are often found on docks or pilings and under rocks, so they are easily accessible to the shallow snorkeler or tide-pooler.  Almost invisible, they generally blend right into their surroundings as they munch away on the acorn barnacles they feed upon.  Sharp radula are used use  to break through the barnacle crust, enabling them to suck out the barnacle’s body.   Look closely for white ribbon-like eggs on docks and often there will be an Onchidoris bilamellata nearby –possible numerous ones.  They are generally about 2.5 cm, but some individuals have been known to reach 3.8 cm.