Definition: Animals are not living things that can be classified as humans. Definition: Multicellular, eukaryote animals of the Kingdom Animalia. An animal (singular: animals) is any of the multicellular, eukaryote animals of the kingdom Animalia (usually characterized to be motile, heterotrophic, having no intercellular nucleus, formed intracellularly, etc.). Thus, an animal can be either a bovine or a dolphin, but it cannot be both. Although most taxonomists agree on the meaning of animals, there are many points of contention concerning the classification of animals. Thus, a definition of an animal can be very vague.
Animals can have sexual or asexual reproduction processes, depending on their mode of development. Sexual reproduction involves both male and female reproductive cells. In both animals, sperm and eggs are produced in a woman’s ovary and fertilized in the man’s semen. The fertilized zygote is then engulfed by the egg and the two are joined in the early part of development. Sexual reproduction in animals is incomplete without the involvement of a partner.
Sexual reproduction involves a process where one or many eggs are released from an ovary. The fertilized egg is enveloped by a protective fluid from the mother’s uterus and travels through the fallopian tube and into the waiting uterus of a woman. The fertilized egg is carried to the fallopian tube by the female sex cells. In animals, however, the fertilized egg is not enveloped by the female’s sex cells; it is immediately expelled after death. However, some animals may live for several days after the death of the sperms.
The other type of reproduction mentioned above is mitogenesis, which refers to the reproduction of multicellular organisms, i.e., single cells. Multicellular organisms are different from single-celled animals because they have a plastid, a germ cell, and multicellular parts such as an eye or a plant stem. Multicellular organisms reproduce sexually. For the sake of simplicity, we refer to all multicellular organisms as animals.
Insects are considered the simplest animals. Unlike animals that reproduce sexually, insects reproduce asexually, by means of sexual division. Insects reproduce asexually by means of a process called independent conception, where the genetic material from an adult insect’s body (rhizome) is used to create a gamete, a small embryo that contains the genetic material needed to reproduce. There are about 400 different species of insects, making the insect kingdom altogether number one on the list of life organisms. The three primary classes of insects include arthropods (which includes moths, spiders, and flies), cicadas (which include ants, bees, and wasps), and termites.
Archaeopteryx is the name for the earliest known animal. Archaeopteryx was a small reptile-like creature covered with a plumage. Modern Archaeopteryx is a three jawed creature that lives in south eastern Asia. Other animals on the mammal category include fishes, eels, tadpoles, chiras, snails, and crabs.