How Do Jellyfish Survive?

There is a reason for everything all creatures great and small. Jellyfish survive by eating as they swim, but beware, their sting is highly poisonous.  One person was trying to cross the english channel, but had to give up, because a jellyfish was following close by and she had to be pulled out of the water for her own safety.  Mind you, she later made another attempt and succeeded.  It is the same old dilema, why do fly's exist, there is a little poem, when got created the fly, he forgot to tell us why.  Everything in nature survives for a reason and serve a purpose to help the ecology of this planet to hopefully stay in balance.  Unfortunately, humans are the biggest culprits when it comes to taking from the land, and not returning