Please help! Which of the following is characteristic of all chordates but NOT found in invertebrates?

a) A circulatory system
b) A notochord
c) An exoskeleton
d) Four legs

Agene pool: a) is the collection of all the alleles found in a single species b) is the collection of all the alleles found in a single population c) is the collection of all the chromosomes found in a single population d) is the collection of genes in a species that allow the individuals to swim