Whose life cycles are the most difficult?

The most complex cycles are periods that include two or more generations of objects of study. For example – known to biologists, marine cnidaria (alternation of all generations of polyps and jellyfish), as well as for representatives of trematodes (a class of parasitic harmful flatworms), their life cycle usually has more than three generations. Representatives of this class are typical representatives with the most complex life cycles. But perhaps all representatives of this class are still unknown to science.