Which statement best explains why invertebrates often have outer skeletons but vertebrates do not?
The outer skeleton provides protection, since invertebrates do not have backbones.
The outer skeleton surrounds the spinal cord, since invertebrates do not have backbones.
The outer skeleton helps invertebrates maintain homeostasis, since they lack spinal cords.
The outer skeleton helps invertebrates maintain homeostasis, since they lack membrane-bound organelles.