Taxonomically, animals belong to the kingdom Animalia, which is one of several kingdoms of living beings. Although there is disagreement on how to best classify the various forms of life on Earth, other major groups of living beings include the bacteria, protists, fungi, and plants. The traits that define the kingdom Animalia are:
Mobility. With few exceptions, such as the sea lily (class Crinoidea), animals are able to freely move about their habitats and are not attached to a substrate.
Multicellularity. Animals begin their lives as a single fertilized ova that multiples into many cells, which differentiate into specialized cells and tissues. Animal body plans are the result of genes known as Hox genes.
Heterotrophism. Animals cannot produce their own source of energy and must consume organic material synthesized by plants.
Sexual reproduction. Reproduction in animals depends on meiosis, or the reduction division of the number of chromosomes from a double set in the adult to a single set in the ova in the female and sperm in the male. The ova and sperm merge to form a cell with the double set of chromosomes that eventually matures into an adult.
Eukaryotic cell. All animals are eukaryotes; that is, each cell has a membrane-enclosed nucleus that contains a double set of chromosomes, one set from each parent.
Death. Animals develop from an embryo that matures into an adult and eventually undergoes programmed death.
There are between 30 and 35 phyla of animals. There are an estimated 1 to 2 million species of animals, 98% of which are invertebrates, and more species are discovered every year. At least half of the invertebrates are insects. By contrast, there are about 4,500 species of mammals, 300 of which are primates. In addition to the animal phyla of insects, the Arthropoda, other examples include the Chordata, or vertebrates, such as mammals; Mollusca, such as clams; Cnidaria, such as corals, Echinodermata, such as sea stars; Platyhelminthes, such as flatworms; and Nematoda, such as round worms. Animals in their natural habitat lead well-ordered lives in the process of finding of adequate food and other resources, reproducing, and, in some cases such as mammals, raising their young.
In addition to the scientific and taxonomic definition of animals as discussed above, there is also the cultural discourse that concerns animality versus humanity. The basic dichotomy is between Western cultures that view the difference between animals and humans as a difference of kind and Eastern cultures that view this difference as one of degree. As a consequence of the attitude on the part of Westerners toward animals, a societal-wide consensus on questions of the value of animals, the extent of their suffering during biomedical experimentation, and animal consciousness and self-awareness are lacking. The debate concerning animal rights and related movements center on these questions of the value and suffering of animals. To be told you are “acting like an animal” does not really mean that one’s actions are animal-like; the comment actually means that the person is acting outside of the bounds of the moral behaviors expected of a person. The debate over the place of animals in our culturally defined worldview is a deeply emotional subject with religious implications for many people, East and West, and one that is unlikely to abate any time soon.
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