#dendrogaster

Sacculina-- group: barnacles, host: crabs

Cancrion-- group: isopod, host: crabs, larva and adult males pictured

Cymothoa-- group: isopod, host: fish, the "tongue eating louse"

Cyprids-- larval stage of barnacles

Nauplius-- larval stage of many crustaceans

Dendrogaster-- closest group: barnacles, host: seastars

Lernaea-- group: copepods, host: fish, the "anchor worm"

Anelasma-- group: barnacles, host: lantern sharks

Lepeophtheirus-- group: copepods, host: fish

Ommatokoita-- group: copepods, host: greenland sharks

Argulus-- group: fish lice, host: fish

The groups I've labelled these as are pretty broad and a lot more complicated when you get into the details, but rest assured they are all currently types of crustaceans! Anchor worms and Argulus fish lice are the species most people would be most likely to encounter as they are common in the aquarium hobby. The others here are a bit more specialized and mostly marine. I also used some generic barnacles as filler, but it should be noted that not all barnacles are considered parasitic.