We all have watched a documentary about lions, tigers, cheetas, sharks... they are big, powerful and astounding living beings that amaze humans because of their size, strength, and characteristics as predators. That's why, sometimes, other equally amazing creatures go unnoticed because they are difficult to find or to see.
This said, I'm going to give a glimpse of how widely vary life cycles of animals with the example of echiurans.
Echiurans are simple animals with a vermiform (elongated, worm-like) body that live in marine or brackish-water habitats. They represent no harm for humans. Most of them lie with the trunk buried in the substratum and feed on detritus that falls on the sea bottom thanks to their proboscis.
The most interesting fact about these animals is their life cycle.
In some species, the female, which may be from a few to as many as 2 meters, is much larger than the male, which is 1-3 mm long. The female release sexually unidfferentiated larvae into the water. The determination of the sex of the larvae depends on where they land! Those that settle on the proboscis of a female, become males that parasite the female, living on their body. If they don't, they grow as females.
This curious sexual development is ultimately determined by a hormone produced by female's proboscis.
Biology is fascinating!
Images taken from: Richard C. Brusca, Gary J. Brusca Invertebrates second edition, 2003.