Complex sentence: Experiments on theoretical issues in conditioning have mostly been done on vertebrates, especially rats and pigeons, but conditioning has also been studied in invertebrates, and very important data on the neural basis of conditioning has come from experiments on the sea slug, "Aplysia".
Split into simpler sentences: Experiments on theoretical issues in conditioning have mostly been done on vertebrates, especially rats and pigeons. However, conditioning has also been studied in invertebrates, and very important data on the neural basis of conditioning has come from experiments on the sea slug, "Aplysia".