The animal kingdom is vast, a place where nearly every animal with an animal physiology exists. Animals are multicellular, meaning that they contain multiple organs capable of secreting various types of substances, including blood, hormones, cells, and other components. Animals breathe oxygen from inside of their own bodies and use this oxygen as a fuel source; animals can reproduce sexually, can move around, and ingest food. Virtually all animals are carnivores and most are semi-dormant, although there are some herbivores and parasites among the list.
The classifications of animals fall into four categories: amphibians, which include amphibious fish and reptiles, amphibians and reptiles, ungulates (which includes hooves and forelimbs), and vertebrates, which include mammals (with the exception of whales and dolphins). There are a number of unclassified animals and plants in the animal kingdom, including parasitic arthropods, ciliates (reeving and bony fish), protozoa, mollusks (plankton and aquatic worms), annelids,urchins, and other cephalopods. A number of bacteria and other pathogens belong to the microbial kingdom. All living things are classified according to their physiological systems and how their organs and tissues function.
Humans are a part of the animal kingdom, but not all animals are part of the human biological kingdom. All multicellular organisms, including plants and animals, are considered to be alive because they reproduce by means of a reproductive process called respiration. Animals also have nervous systems, although most people don’t consider them to be “conscious.” A living animal is characterized by the following traits: the ability to feel the presence of other animals or people, the ability to sense temperature, air pressure, and other external factors, the ability to perceive light and sound, the capacity for growth and development, the ability to form tissues and organs, and the ability to adapt to its surroundings. An animal can survive for a very long time if it has an adequate immune system and good nutrition; it must also be capable of communication with other animals and humans, as well as the ability to sense damage to its body and use it as a defense against predators.
In contrast to what many people believe, not all animals are classified in the same way by taxonomy, genetics, anatomy, and physiology. Each classifications of animals is determined by the type of cells that make up that organism, which is dictated by its physical features, its ability to survive, its ability to feed, its ability to reproduce, and other considerations. For instance, although cats are considered to belong in the mammal category, they actually are part of a group of multicellular organisms known as feline leukemia virus (MBV). In fact, cats and dogs are only subtypes of MBV; cats are a special case of a very common virus that affects multicellular organisms.
The other classification of animals is known as eutherian mammals, which includes reptiles, amphibians, mammals which are amphibious and prototheria, which includes non-mammal vertebrates. Multicellular organisms such as bacteria, fungi, plants, and even algae all fall under this broad classification. All of the animals we care to know about are indeed mammals, but they have undergone asexual reproduction, a process which has been missing in these species since before their evolution.
Therefore, if there was no such thing as mammals, then what would be the definition of what an animal species would be? It would certainly be very different from the view that animals can only be classified based on whether or not they reproduce sexually. Without sexual reproduction, there would be no way for animals to be classified as one specific kind, which is what has been done throughout the history of science.