Appendages of a number of hoofed animals. As a rule, horns are made of tough fibrous material called keratin and are not shed. Any individual animal or plant having diverse organs and parts that function together as a whole to maintain life and its activities. The fundamental biological classification, comprising a subdivision of a genus and consisting of a number of plants or animals all which have a high degree of similarity, can generally interbreed only among themselves and show persistent differences from members of allied species. It is important that stu-dents have some understanding of how and why scientists classify animals.
Scientists classify animals according to how closely related they are to each other.
One major group of animals is vertebrates. These animals all have backbones. Vertebrates are divided into smaller groups that include fish, amphibians, reptiles, birds and mammals. A mammal is an animal that feeds its young milk produced in the mammary glands located on the mother. The development of mammary glands separates mammals from all other living things.