Arthropods are a very species rich and very diverse group of animals, but using this system we can safely classify them into several big categories by the type, class and position of its appendix in the list.
In the real life this translates to:
'( antenna leg leg leg)
This is an hexapod, for example an ant, but:
'( fake-antenna leg leg leg)
This is Myrmarachne a spider mimicking an ant. It moves its first pair of legs to simulate an antenna and trick its preys, but we know that the structure is different. If you are an ant, you would be dead by now.
'( antenna antenna antenna leg leg)
This animal does not exist. Not in this planet. Is an alien.
If we have a partial fossil with one leg and a couple of wings, we can say that this was a modern insect. Wings developed only in the modern insects.
The appendix and its location are so different among the main groups of arthropods and so fixed inside the groups that we can safely say that trilobytes didn't fall in any known category of alive arthropod. Those were not crustaceans and all went extinct.
We also know that spiders are not just crustaceans that lose a couple of legs and a couple of antennas. Its legs are placed in different segments, and the auxiliar structures in its head are totally different.
In the real life this translates to:
'( antenna leg leg leg)
This is an hexapod, for example an ant, but:
'( fake-antenna leg leg leg)
This is Myrmarachne a spider mimicking an ant. It moves its first pair of legs to simulate an antenna and trick its preys, but we know that the structure is different. If you are an ant, you would be dead by now.
'( antenna antenna antenna leg leg)
This animal does not exist. Not in this planet. Is an alien.
If we have a partial fossil with one leg and a couple of wings, we can say that this was a modern insect. Wings developed only in the modern insects.
The appendix and its location are so different among the main groups of arthropods and so fixed inside the groups that we can safely say that trilobytes didn't fall in any known category of alive arthropod. Those were not crustaceans and all went extinct.
We also know that spiders are not just crustaceans that lose a couple of legs and a couple of antennas. Its legs are placed in different segments, and the auxiliar structures in its head are totally different.