Unusual Animal Friends

In addition to these, the deuterostomes also include the Hemichordata, or acorn worms. Although they are not especially prominent today, the important fossil graptolites may belong to this group.
The Chaetognatha or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.
• Ecdysozoa
The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis. The largest animal phylum belongs there, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.