The benefit of using Aplysia to study the neural and biochemical processes that allow for classical conditioning in simple organisms is that:

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands

when it is at an intermediate population density

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.