WordNet sense 1 (someone able to acquire new knowledge and skills rapidly and easily):

quick study

WordNet sense 2 (a follower who hangs around a host (without benefit to the host) in hope of gain or advantage):

leech, parasite, sponger

WordNet sense 3 (ask for and get free; be a parasite):

bum, cadge, grub, mooch

WordNet sense 4 (primitive multicellular marine animal whose porous body is supported by a fibrous skeletal framework; usually occurs in sessile colonies):

parazoan, poriferan