Here's the difference:
* Poisonous means that a substance can harm you if you ingest it.
* Venomous means that a substance can harm you if it is injected into your body, usually through a bite or sting.
The Crown-of-Thorns starfish has venomous spines that can inject a toxin into the skin of any creature that touches them. This toxin can cause extreme pain, inflammation, and even death in extreme cases.
So, while the starfish itself isn't poisonous, its spines are venomous and pose a serious threat.