The classic example is the Echiuroid worm, Bonellia . (Giese & Pearse, Invert. Reproduction Vol. III) When a planktonic larva settles down on the normal mud substrate, in isolation, it develops into a female. If the larva happens to find a female worm first, it enters the coelomoduct of the female, differentiates into a male, and lives there as a parasite. The male is very rudimentary, and essentially just becomes a male gonad, nourished by the female, that produces spermatozoa to fertilize the eggs produced by the female. Of course, it differs from a typical male gonad in having a different genotype from the rest of the animal, so the advantages of sexual recombination are preserved. (There may be up to 20 males in one female.) (There have been some experiments showing that female extracts can cause larvae to develop as males.)
Other examples are found in some parasitic isopods, parasitic wasps, some orchids.
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