What are the reasons that you can think of for the arthropods to constitute the largest group of the animal kingdom?

What does radial symmetry means? What is the type of symmetry found in chordates? Which are other phyla of the animal kingdom that present species with radial symmetry?

Which of the following is the correct classification for the following chordate animal: This is a vertebrate that does not have a jaw.  An example is the Hagfish which has a cartilaginous skull and skeleton.   Question 30 options:   a)  Urodela / Salamanders   b)  Eutharia   c)  Superclass Agnatha   d)  Birds    e)  Anura / Frogs   f)  Monotreme   g)  Class Osteichthyes   h)  Class Chondrichthyes   i)  Reptiles (non avian)   j)  Marsupial   k)  Cephalochordata

What are four synapomorphies and four symplesiomorphies of the vertebrate body plan?