Animals are often grouped as
a. vertebrates or invertebrates.
b. heterotrophs or autotrophs.
c. walkers or crawlers.
d. swimmers or runners.

The answer is (a.) vertebrates or invertebrates

There are two main classifications of animals: vertebrates and invertebrates. The difference between the two is the vertebrates have backbones or a spinal column while the invertebrates do not have. A human is an example of a vertebrate and flatworms and other insects are examples of invertebrate.