Add a comma when you have a conjunction such as while, because, since, also, etc. You also add a comma when you are naming multiple things such as milk, bread, and toast. You can also add a comma to separate sentences or run-ons.
In your case, you need to add a comma because "known as a carapace" is a phrase in which it describes the protective shell. They are always enclosed by commas and as you can see at the end of "known as a carapace" there is a comma.