Reproduction is essential for the continuation of a species. Invertebrates have adapted many ways to do this successfully. Some reproduce asexually (only one parent) and others reproduce sexually (use of sperm and eggs). There are even some species of invertebrates that reproduce both sexually and asexually.
The jellyfish, like the one shown here that has washed up on shore, uses sexual reproduction; where male and female jellyfish release a lot of sperm and eggs into the water at the same time. The fertilized eggs grow into larvae that swim off and rest on the bottom of the ocean. The larva grows into a polyp-like structure that then begins to develop asexually, producing many tiny jellyfish (ephyra) that are released into the ocean. The ephyra then grow into adults.
An earthworm (not something we'd find in a tidal pool, but related to some marine worms) is an invertebrate that has a unique reproduction method.
Most worms, the earthworm included, reproduce sexually. That is, an egg must be fertilized by a sperm.
What's unique about the earthworm is that each individual worm has both male and female reproductive organs. So it produces both egg and sperm. We call organisms like this hermaphrodites. Although they can not fertilize their own eggs, the advantage is that any worm is a potential mate for another worm. As earthworms mate, sperm is deposited into each worm and eggs can be fertilized. When you are a slow mover, like a worm, this works to your advantage.