Here's why:

* Eggs and sperm are released into the water: Starfish reproduce sexually by releasing eggs and sperm into the surrounding water.

* Fertilization occurs externally: The eggs are fertilized externally, meaning the sperm meets the egg outside of the female's body.

* Larvae develop in the water: After fertilization, the fertilized eggs develop into free-swimming larvae that live in the plankton and feed on microscopic organisms.

* Metamorphosis into adult form: Over time, the larvae undergo metamorphosis, gradually developing into the familiar adult starfish form.

This entire process of development takes place outside of the starfish's body, making it external development.