While feather stars and sea lilies are both crinoids (belonging to the class Crinoidea), they are not the same thing. They are two different *subclasses* within the class. Here's the breakdown:

Feather stars (subclass Comatulida)

* Mobile: They have a stalk in their juvenile stage, but they detach and become free-swimming as adults, using their arms for locomotion.

* Arms: They have flexible arms that are branched and feathery, giving them their name.

* Habitat: They are found in shallow, coastal waters, usually clinging to rocks, coral, or seaweed.

Sea lilies (subclass Articulata)

* Sessile: They remain attached to the substrate throughout their lives using a long, flexible stalk.

* Arms: They have long, unbranched arms that are not as feathery as feather stars.

* Habitat: They are typically found in deep water, attached to rocks or the seabed.

So, while both are crinoids, they differ in their mobility, arm structure, and preferred habitats.