Animalia: Characteristics, Functions, and Classification
In the scientific classification of living beings, the kingdom Animalia (animals) or Metazoa (metazoans) is a broad group of eukaryotic, heterotrophic, multicellular, and tissue-based organisms. They are characterized by their capacity for locomotion, the absence of chlorophyll and cell walls, and their embryonic development, passing through a blastula stage and determining a fixed body plan (although many species can subsequently undergo metamorphosis). Animals are a natural group closely related to fungi and plants. Animalia is one of the five kingdoms of nature, and it includes human beings.
Characteristics
Mobility is the most striking feature of the organisms in this kingdom, but it is not exclusive to the group, which results in certain organisms such as those belonging to the kingdom Protista often being referred to as animals.
The following scheme shows the characteristics common to all animals:
- Cellular organization: Eukaryotic and multicellular.
- Nutrition: Heterotrophic by ingestion (at the cellular level by phagocytosis and pinocytosis), unlike fungi, which are also heterotrophs but absorb nutrients after digesting them externally.
- Metabolism: Aerobic (consuming oxygen).
- Reproduction: All animal species reproduce sexually (some only by parthenogenesis), with gametes of very different sizes (oogamy) and zygotes (diplontic life cycle). Some may also multiply asexually. They are typically diploid.
- Development: By embryo and embryonic germ layers. The zygote divides repeatedly by mitosis to give rise to a blastula.
- Structure and functions: Have collagen as a structural protein. Highly differentiated cell tissues. No cell walls (some with chitin). Phagocytosis in basal forms. Ingestion with phagocytosis and subsequent absorption in derived forms (“more evolved”), capable of movement, etc.
- Symmetry: Except for sponges, other animals show a regular arrangement of body structures along one or more body axes. The main types of symmetry are radial and bilateral.
With few exceptions, most notably sponges (phylum Porifera), animals have differentiated and specialized tissues. These include muscles, which can be contracted to control movement, and a nervous system, which sends and processes signals. There is usually also an internal digestive chamber, with one or two openings. Animals with this type of organization are known as eumetazoans, as opposed to Parazoa and Mesozoa, which have simpler levels of organization and lack some of the features mentioned.
All animals have eukaryotic cells, surrounded by an extracellular matrix composed of characteristic collagen and glycoprotein bands. This can become calcified to form structures like shells, bones, and spicules. During the development of the animal, a relatively flexible framework is created by which cells can move and reorganize, making more complex structures possible. This contrasts with multicellular plants and fungi, which develop through progressive growth since their cells remain in place due to cell walls.
Essential Functions
Animals carry out the following functions: feeding, respiration, circulation, excretion, response, movement, and reproduction:
- Food: Most animals cannot absorb food; they ingest it. Animals have evolved in different ways to eat. Herbivores eat plants, carnivores eat other animals, and omnivores eat both plants and animals. Detritivores eat decomposing plant and animal material. Filter-feeders are aquatic animals that strain tiny organisms floating in water. Animals also form symbiotic relationships, in which two species live in close association with one another. For example, a parasite is a type of symbiont living in or on another organism, the host. The parasite feeds on the host and damages it. 1
- Breathing: No matter if they live in water or on land, all animals breathe, which means they take in oxygen and discharge carbon dioxide. Some animals with very simple bodies and thin walls use diffusion of these substances through the skin. However, most animals have evolved complex tissues and organ systems for respiration. 1
- Circulation: Many small aquatic animals, such as some worms, use only diffusion to carry oxygen and nutrients to all their cells and collect waste products. Diffusion is sufficient because these animals have a thickness of only a few cells. However, larger animals have some type of circulatory system to move substances around inside their bodies. 1
- Excretion: A primary waste product of cells is ammonia, a poisonous substance containing nitrogen. The accumulation of ammonia and other waste products could kill an animal. Most animals have an excretory system that eliminates ammonia or transforms it into a less toxic substance that is eliminated from the body. By eliminating metabolic waste, the excretory system helps maintain homeostasis. Excretory systems vary, from cells that pump water out of the body to complex organs such as kidneys. 1
- Response: Animals use specialized cells called nerve cells to respond to events in their environment. In most animals, nerve cells are connected to form a nervous system. Some cells, called receptors, respond to sounds, light, and other external stimuli. Other nerve cells process information and determine the animal’s response. The organization of nerve cells within the body changes dramatically from one phylum to another.1
- Movement: Some adult animals remain fixed in one place. While many have motility, i.e., they move. However, both fixed and motile animals typically have muscles or muscle tissues that shorten to generate force. Muscle contraction allows animals to move, often in combination with a structure called a skeleton. Muscles also help animals, even the most sedentary, to eat and pump water and other fluids through the body. 1
- Reproduction: Most animals reproduce sexually by producing haploid gametes. Sexual reproduction helps create and maintain the genetic diversity of a population. Therefore, it helps to improve the ability of a species to evolve with a changing environment. Many invertebrates can also reproduce asexually. Asexual reproduction gives rise to offspring genetically identical to the parents. This form of reproduction allows animals to increase rapidly in number. 1