Question #e9b7e
1 Answer
Because the distinguishing characteristics of the phylum Arthropoda are: exoskeleton and jointed appendages, none of which are present in Nematodes (roundworms) or in Annelida (segmented worms).
Explanation:
First of all, the term for "sub. Kingdom" is Phylum (phyla is the plural), so from now on, whenever I mention phylum, just know that's the term used for sub. Kingdom.
Now that we have that out of the way, let's see why these two phyla (Nematoda and Annelida) can't be under the the phylum arthropoda.
There are similarities and differences between these phyla. Let's start with similarities:
Animals and their phyla are typically classified into two big groups; Asymmetrical animals and symmetrical animals. Symmetrical animals then break into two other groups, which are Radially symmetrical, or Bilaterally symmetrical.
All three of the phyla in your question are a part of bilaterally symmetrical group of animals, which also mean they're triploblastic.
Now that we know how they are similar, let's see the differences and why Nematoda (roundworms) and Annelida (segmented worms) can't be under the phylum Arthropods:
Two distinguishing factors of the phylum Arthropoda are their exoskeleton (made of chitin) and jointed appendages ( e.g. wings, legs, antennae), and neither of those are present in round or segmented worms.
As a matter of fact, even Nematoda and Annelida can't be under the same phylum, despite the fact that they're both worms, because they each have unique characteristics (Nematodes lack a circulatory system and they only have longitudinal muscles which causes a thrashing movement, but Annelida/segmented worms do have a circulatory system).
Now it should be a little clearer to see why those two phyla can't be under Arthropoda. Or why it is hard to even put the round and segmented worms together in one phylum. They all have distinguishing characteristics.
Hope it helped (c:
