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Characteristics of Animals

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic (having cells with a nucleus and organelles inside them) and mostly multicellular, which separates them from bacteria and most protists. Animals are also heterotrophic, generally digesting food within an internal chamber, which separates them from plants and algae. They are distinguished, as well, from plants, algae, and fungi by lacking rigid cell walls. All animals are motile (having the ability to move independently), if only during a certain stage of life. In most animals, embryos go through a blastula stage (stage of hollow ball of cells), which is also a characteristic exclusive to the animal realm.

Structure of Animals

With few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies that are differentiated into separate tissues. These tissues include muscles (able to contract and control locomotion) and nerve tissues (send and process signals). There is usually also an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.

All animals have eukaryotic cells, or cells with an internal nucleus, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This matrix gives the cell flexibility, allowing soft but distinct tissues to form. In some animals, this matrix may be calcified to form structures such as shells, bones, and spicules (spiny structures). During the development of the cells, the extracellular matrix forms a relatively flexible framework upon which other cells can move about and be reorganized, making complex structures possible. By contast, other multicellular organisms such as plants and fungi have cell held in place by cell walls, and so they develop a progressive, more rigid growth. Likewise, unique to animal cells are the following intercellular junctions (multiprotein complexes that allow contact between neighboring cells or between a cell and the extracellular matrix):

• Tight junctions - These act as barriers that regulate the movement of water and solutes between epithelial layers.
• Gap junctions - Also called communication junctions, these allow for direct chemical communication between adjacent cellular cytoplasm through diffusion without contact of the extra cellular fluid.
• Desmosomes - Also known as a macula adherens, these are a type of cell structure specialized for cell-to-cell adhesion. Desmosomes can be though of as rivets through the plasma membrane of adjacent cells.

Reproduction & Development

Nearly all animals experience some form of sexual reproduction. They have specialized reproductive cells which undergo meiosis to produce smaller, motile spermatozoa (sperm) or larger, non-motile ova (eggs). In sexual reproduction, these cells fuse to form zygotes which develop into new individuals.

Many animals are also capable of asexual reproduction, or reproduction by a single individual. This may take place through parthenogenesis (where fertile ova are produced without mating), budding, or fragmentation. A zygote initially develops into a hollow sphere (a blastula) which then encounters rearrangement and differentiation. In sponges, blastula larvae swim to a new location and become new sponges. In most other groups, the blastula sustain more complicated rearrangement. It first invaginates to form a gastrula (a hollow cup-shaped structure with three layers of cells) with a digestive chamber, and two separate germ layers, an external ectoderm (outer-skin) and an internal endoderm (inner-skin). In most cases, a mesoderm (middle-skin) also develops between them. Once in place, these germ layers will differentiate to form the tissues and organs of the animal.

Food & Energy Sourcing

All animals are heterotrophs, which means that they feed directly or indirectly on other living things. They are often further subdivided into groups based on their feeding and eating habits, such as carnivores, herbivores, omnivores, and parasites.

Predation is a biological interaction where a predator feeds on its prey. Predators may or may not kill their prey prior to feeding on them, but the act of predation will always result in the death of the prey. The other main category of consumption is detritivory, or the consumption of dead organic matter. At times it can be difficult to separate the two feeding behaviors, such as where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between predator and prey, thus resulting in various antipredator adaptations.

Most animals feed indirectly from the energy given off by sunlight. Plants use this energy to covert sun rays into simple sugars during photosynthesis. Starting with the molecules carbon dioxide (CO2) and water (H2O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of glucose (C6H12O6) and releases the oxygen (O2) back into the atmosphere. These sugars are then used as building blocks that allow the plant to grow. When animals eat these plants (or other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or are broken down, releasing the stored solar energy, and giving the animal energy it needs to move. This process is called glycolysis. Animals living close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the Sun's energy. Instead, at the bottom of the ocean, chemosynthetic archaea and bacteria form the base of the food chain.

If I may, let me take a moment here to point something out: In the way that plants make their food, they are practically magical. As far as I know, plants are the only natural creatures that can take something with absolutely no weight (light photons) and convert them into physical, weighted creations (sugars). Plants have been doing something for millions and millions of years that it has taken humans about 200,000 years to artificially replicate - turning light into solar energy. But even we haven't figured out how to just take light out of the sky and turn it into food - we've only turned it into electricity, which isn't as big of a conversion that plants are undertaking at a much more efficient cost-profit ratio. As Arthur C. Clarke reminds us, "Any sufficiently advanced technology is indistinguishable from magic." Bravo, plants!