Many have complex life cycles that reach an apex in some which must pass through as succession of well defined hosts. A platyhelminths is a flatworm helminth. A cestodes is a tapeworm platyhelminth. They attach at one end to the wall of their host but otherwise do not absorb nutrients using a specialized organ. Trematodes are fluke platyhelminths.
Nematodes are roundworms.
Nematodes do not have nearly as complex life cycles as do platyhelminths. For some nematodes the egg is infective while for others it is the larva that are infective. Questions No entry. An Introduction .