Mammals are vertebrate animals belonging to the domain Eukaryota, Kingdom Animalia, and Filo Chordata. They are subdivided into two groups: aquatic, and terrestrial. The main feature that differentiates mammals from other classes is the presence of mammary glands, which produce milk for feeding offspring. They are endothermic animals - that is, warm blooded, and their temperature is regulated internally. They are believed to have emerged about 170 million years ago in the Jurassic period.