Re: Do starfish have an exoskeleton? Or is it considered a shell?

Hi there

Good question. Starfish (called Asteroids by scientists) do indeed have an 
exoskeleton. It's not quite as obvious as the one on a crab, but it is 
still considered an ecoskeleton.

If we were to cut down into a starfish, we'd go through the tough outer 
skin, then through a layer of ossicles - small bony fused plates that form 
a tough barrier to the outside world, and also something for the seastar's 
hydraulic movement systems to push and pull against (much the same as your 
muscles pushing/pulling against your bones to make your arms and legs 
move).

Further down, you'd find a simple radial nervous system and then the 
amazing hydraulic system where seawater is moved around to make the arms 
of the seastar move, as well as the tiny little legs (called tube feet) 
move.

Soft bodied animals like snails have shells - most other marine 
invertebrates have exoskeletons, even the soft bodied sea cucumbers, only 
it is just greatly reduced.

Hope this helps.

-Alastair