Most animals without backbones, known as invertebrates, have no tails. Among these are spiders, most insects and frogs. However, every species of mammal has a tail at some point in its development. Human embryos possess tails during their fifth to eighth week of development. In rare cases, humans are born with imperfectly developed tails, which must be removed by surgery.
In vertebrates, or animals with backbones, tails are usually quite distinctly visible and used for a specific evolutionary purpose. Domestic mammalian pets like dogs and cats have very distinct tails, as do most wild mammals. Some mammals have tails that are less pronounced, such as guinea pigs, but they are still there.
However, in many other species, what is considered a tail is not so clear. For example, crickets and many other insects have posterior hairs in the place where a tail would be. Crayfish possess a series of five "plates," which are used to propel the animal backwards through water. The anatomical placement of these structures on many animals suggests that evolution has provided some species with an extension of the vertebrae that is not quite a tail but that tends to serve a similar purpose to a tail.