Animals are often defined as organisms that are:
multicellular (made up of many cells),
heterotrophic (feeding on others),
diploid (with two sets of chromosomes), that
reproduce sexually, developing from the fusion of an egg and a sperm cell, and that
follow certain stages during their embryonic development: a morula, followed by a blastula (a multicellular embryo that develops from the zygote produced by fertilization of a large egg by a smaller sperm), and finally a gastrula (a hollow sac that forms the embryonic precursor to the digestive tract, by means of which animals ingest nutrients and excrete waste) (World Biodiversity Database, 2000).
This definition is both too broad and too narrow. Many other kinds of eukaryotic organisms are also multicellular - e.g. plants, fungi and slime moulds. Fungi and many protoctista are also heterotrophic, while plants develop from embryos that result from the sexual fusion of a sperm and an egg. These features, then, are not unique to animals. On the other hand, development from a blastula is unique to animals, but a few animals (certain sponges) do not develop in this way (World Biodiversity Database, 2000).
The only feature that appears unique to animals is: a special kind of extracellular matrix (ECM - the substance between cells) that is composed of four types of molecules - collagen, proteoglycans, adhesive glycoproteins, and integrin - that are created inside, but exist outside, the cells of animals. Extracellular matrix plays a key role in the development of animals. It guides the development of mobile cells in the developing embryo, and also helps to control the transition of cells from one type to another. (Mobile cells enable animals to develop from single cells into working individuals composed of many cells.) It appears that all animals share this complex pattern of development mediated by their extracellular matrix, whereas all other multicellular organisms do not (Morris, 1993).
Animals exhibit different kinds of sensory behaviour, such as attraction to light, avoidance of noxious chemicals, and the ability to sense dissolved gases and temperature. Such behaviour is found in members of all five kingdoms of living things, but animals have most elaborated this theme (World Biodiversity Database, 2000). Generally, animals' senses can be classified into three groups: electromagnetic senses (which detect changes in light, heat, and other kinds of electromagnetic radiation); mechanical senses (which respond to sound, touch, gravity, and stretching); and chemical senses (taste and smell).
There is a fundamental difference between Aristotle's teleological definition of animals, and the contemporary scientific definition. For Aristotle, as we have seen, the existence of sensory capacities was a defining characteristic of animals: without these discriminatory abilities, animals could not survive, avoid danger or acquire what they need (De Anima 3.9, 432a16, 3.12, 434a30 - 434b3). Locomotion was a sufficient but not a necessary condition for being an animal (De Anima 3.9, 432a16; 3.9, 432b19-20, 3.12, 434b8).
By contrast, the modern scientific definition of "animal" is based on the fundamental similarities in their structure and bio-molecules - especially at the cellular level - between organisms that were formerly classified as animals on the basis of their sensory capacities. It is at the cellular level that animals look most alike (Morris, 1993). Because the criterion that defines animals is now based on their biochemistry and not their psychology, certain organisms that might not have been previously regarded as animals (owing to their lack of perception) can now be classified as animals, while organisms whose biochemistry is too different from that of animals can be relegated to another kingdom, sensory or locomotive capacities notwithstanding.
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