Animals are multicellular, eucalyptus-like organisms in the kingdom Animalia. Although there is great diversity across the animal kingdom, there are six subkingdoms or domains of animal life. The categories of animals include fishes, amphibians, reptiles, mammals, birds, insects, and crustaceans. Although there are unicellular organisms such as bacteria, some of the categories of animals such as plants also belong to a class of unicellular organisms. With few exceptions, all animals eat organic matter, breathe air, can move, can reproduce sexually, and have their own distinctive characteristics unique to their kind.
All animals reproduce through a process called sexual reproduction. Sexual reproduction involves producing offspring by combining genetic material from one partner with that of another. In the animal kingdom, sexual reproduction takes place through a series of events beginning with fertilization where an egg is fertilized by a sperm. The fertilized egg is then placed into an ovary where it undergoes development before it is released into the fallopian tubes for fertilization by a sperm. Sexual reproduction in animals is completed when the egg and sperm cells divide and the resulting offspring are born.
The reproductive process may be slow or rapid, depending on the animal, its environment, its age, its characteristics, and its ability to reproduce. The reproductive process in animals may be separated into two broad categories, asexual reproduction. Among asexual reproduction, some animals such as insects reproduce by a process called chemosynthesis, while others, such as the algae, are capable of living on just a layer of minerals.
Plants and animals are classified according to the life cycles they go through during a year. The different life cycles include respiration, photosynthesis, regeneration, fertilization and birth. Plants are classified according to the type of respiration that they have, whether it be air, water, wind or sunlight. Some plants require specific types of light to survive while others do not. Animals including fishes, birds, lizards and snakes all breathe through their gills. Fishes such as the swordfish do not breathe through their gills, but rather via gills that connect to the upper part of the ocean, called the diaphragm.
A variety of forms of life form on earth include prokaryotes, bacteria, fungi, protists, single-celled algae, methanoplankton, unicellular eukaryotic organisms and fungi. Plants belong to the kingdom Animalia, the most common animal kingdom in the entire planet Earth. A variety of animals constitute the Protista kingdom animalia, including such familiar creatures as cows, dogs, cats, rats and birds. The diverse life that lives on Earth includes unicellular eukaryotic organism, photosynthesizing algae, aerobic unicellular eukaryote and photosynthetic cyanobacteria.
The classification of animals is based on the arrangement of their organs and the physiology of those organisms. Anatomy and physiology are studied in order to group different species. The classification of animal species is also based on the relationships among those species. Humans belong to the Kingdom Animalia, so all of their species are grouped in this branch. Within the Kingdom Animalia there are five subkingdom levels which are: Prototheria, Ungulates, Multicellular Eukaryotes and Plantae.