<P> The integumentary system is made up of three layers: the outermost epidermis, the dermis and the hypodermis . The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer . Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward . The middle layer, the dermis, is 15 to 40 times thicker than the epidermis . The dermis is made up of many components, such as bony structures and blood vessels . The hypodermis is made up of adipose tissue, which stores lipids and provides cushioning and insulation . The thickness of this layer varies widely from species to species; marine mammals require a thick hypodermis (blubber) for insulation, and right whales have the thickest blubber at 20 inches (51 cm). Although other animals have features such as whiskers, feathers, setae, or cilia that superficially resemble it, no animals other than mammals have hair . It is a definitive characteristic of the class . Though some mammals have very little, careful examination reveals the characteristic, often in obscure parts of their bodies . </P> <P> Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material . To break up intact plant tissues, mammals have developed teeth structures that reflect their feeding preferences . For instance, frugivores (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low - crowned teeth specialized for grinding foliage and seeds . Grazing animals that tend to eat hard, silica - rich grasses, have high - crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low - crowned teeth . Most carnivorous mammals have carnassialiforme teeth (of varying length depending on diet), long canines and similar tooth replacement patterns . </P> <P> The stomach of Artiodactyls is divided into four sections: the rumen, the reticulum, the omasum and the abomasum (only ruminants have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material . The solids lump together to form a bolus (or cud), and is regurgitated . When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again . Ingested food passes to the rumen and reticulum where cellulytic microbes (bacteria, protozoa and fungi) produce cellulase, which is needed to break down the cellulose in plants . Perissodactyls, in contrast to the ruminants, store digested food that has left the stomach in an enlarged cecum, where it is fermented by bacteria . Carnivora have a simple stomach adapted to digest primarily meat, as compared to the elaborate digestive systems of herbivorous animals, which are necessary to break down tough, complex plant fibers . The caecum is either absent or short and simple, and the large intestine is not sacculated or much wider than the small intestine . </P> <P> The mammalian excretory system involves many components . Like most other land animals, mammals are ureotelic, and convert ammonia into urea, which is done by the liver as part of the urea cycle . Bilirubin, a waste product derived from blood cells, is passed through bile and urine with the help of enzymes excreted by the liver . The passing of bilirubin via bile through the intestinal tract gives mammalian feces a distinctive brown coloration . Distinctive features of the mammalian kidney include the presence of the renal pelvis and renal pyramids, and of a clearly distinguishable cortex and medulla, which is due to the presence of elongated loops of Henle . Only the mammalian kidney has a bean shape, although there are some exceptions, such as the multilobed reniculate kidneys of pinnipeds, cetaceans and bears . Most adult placental mammals have no remaining trace of the cloaca . In the embryo, the embryonic cloaca divides into a posterior region that becomes part of the anus, and an anterior region that has different fates depending on the sex of the individual: in females, it develops into the vestibule that receives the urethra and vagina, while in males it forms the entirety of the penile urethra . However, the tenrecs, golden moles, and some shrews retain a cloaca as adults . In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally . Monotremes, which translates from Greek into "single hole", have a true cloaca . </P>

All mammals female give birth to live young