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Members of the biological kingdom of Animalia are animals. Animals are eukaryotic organisms that are generally multi-cellular, oxygen-dependent, heterotrophic, motile, and sexually reproductive. The cells of an animal do not have a rigid cell wall and, like other eukaryotic organisms, have organelles from folded membranes. Animal zygotes form a blastula during the development of an embryo to aid in the development of organs and other body structures. There are millions of estimated species of animals in the world and most of them are predicted to be insects. Animals are a major part of most of the ecosystems on the planet forming the majority of the food-webs in these ecosystems. Many species of animals spend at least a portion of their lives in soil.


The outer cells of a blastula are the blastomeres, they surround a fluid filled cavity surrounding the blastocoele. This is an embryo of an animal cell in its early stages.

Animals are multi-cellular organisms and, with the certain exceptions, these cells form specialized groups called tissues. These tissues have many different functions including, locomotion, structure, digestion, and various other processes necessary for life.[2} These tissues begin to be created when the blastula is formed. The blastula is a hollow sphere of zygotic cells that form after repeated division of fertilized animal egg. This hollow sphere begins the formation of the internal organs and tissues. A non-insignificant portion of tissues is not comprised of cells, they have a large amount of extracellular space in them. This space is mostly filled by the extracellular matrix. The extracellular matrix is comprised of macromolecules such as proteins and polysaccharides. These molecules are produced by the cells in the tissues and they also arrange them within the tissues. The extracellular matrix is more prevalent in connective tissue and can calcify to form bones and similar structures.[1]

Animals are heterotrophic, they can not produce their own nutrients for aerobic respiration. Like fungi, animals must consume complex carbon molecules for aerobic respiration. These complex nutrients can be acquired by the from other organisms, live or dead. Detritovores consume the tissues of dead organisms. Grazers may consume parts of still living organisms, although these organisms are herbivores, primarily consuming plant matter. Predators kill and consume other organisms, mainly animals. Parasites and parasitoids both grow and develop either within or on other organisms, taking nutrients from them. Parasitoids, however, kill their hosts. [4]

An animal cell with the organelles labeled. Notice the lack of a cell wall in the animal cell

The cells of animals are eukaryotic, meaning they are surrounded by a cell membrane and have a nucleus with all of the genetic data of the cell inside. The biggest difference between animal cells and the cells of plants and fungi is that animal cells lack a cell wall. This lack of a cell wall allows for the differentiation of cells and the production of tissues. Most animal cells are diploid, therefore chromosomes exist in homologous pairs.These pairs break up during mitosis, and during meiosis these cells cross-over and then divide to form haploid cells. The organelles within an animal cell are mostly made up of membranes, or in the case of mitochondria, the remnants of a symbiotic bacteria. Certain organelles are found more often in animal cells, such as the centrioles which aid in the process of cell division. [3]

Taxonomy and Diversity

There are roughly 1.7 million described species in the animal kingdom. These animals are spread across 40 different phyla. Arthropods comprise roughly 1.3 million of these species and are by far the most numerous among them representing about 80% of the kingdom. The next largest phylum are the mollusks, representing about 120,000 species and having less than 10% of the diversity of the arthropods. The phylum Craniata includes all vertebrates representing 85 thousand species. 35 thousand of these species are fish, 16 thousand are mammals, 15 thousand are reptiles, 11 thousand are birds, and 7 thousand are amphibians. [5]

Phylogenetic tree of the major phyla of the Animal kingdom

Animals in Soil

Animals play a large role in the soil ecosystem. Invertebrates mix, loosen, and aerate soils, while the larger vertebrates burrow in the soil. Their organic materials such as their wastes and their decaying bodies are important to the replenishment of nutrients in the soil.

A pair of honey badgers fending off a pack of African wild dogs

There are multiple ways to classify animals that live in the soil. Size is one of the primary ways to classify these animals. Microfauna are between 0.0002 cm to 0.002 cm in body length, mesofauna are from 0.002 cm to 1 cm in length, and macrofauna are over 1 cm in body length. Another classification is the span of time an animal resides in soil, from animals like nematodes, that spend their entire lives in soil, to animals like ground nesting birds which tangentially exist on the soil. The animals location in the soil horizons, the diet of the animal, and the method of locomotion through the soil are also ways to classify soil based fauna.

Both vertebrates and invertebrates effect the communities in soil. Nematodes are detritovores that also consume bacteria and fungi and can be parasitic to other invertebrates and plants. They are an important food source to other soil based animals. Earthworms mix the organic and inorganic aspects of soil, and alter nitrogen into more consumable forms for plant life. Mollusks and arthropods are both important for cycling surface plant matter underground. Vertebrates are important for the topography of the soil. Many of the mammals burrow and alter the soil horizons of a given area. Small rodents store food in these burrows, which eventually wind up back in the soil with their droppings. Nutrients may enter the soil via droppings from surface animals and large animals such as cattle may overgraze making the soil more susceptible to wind erosion. The burrows of some animals may become the homes of some organisms that don't normally live in the soil such as certain species of beetles and frogs. Fungi would then start growing which would then be consumed by arthropods which would then be consumed by vertebrates.[6]


1. Davidson, Michael W. "Animal Cell Structure". Archived from the original on 20 September 2007. Retrieved 20 September 2007.

2. Starr, Cecie (2007-09-25). Biology: Concepts and Applications without Physiology. Cengage Learning. pp. 362, 365. ISBN 0495381500.

3. Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002). Molecular Biology of the Cell (4th ed.). Garland Science.

4. Britannica, T. E. (2017, June 19). Heterotroph. Retrieved March 06, 2018, from

5. Zhang, Zhi-Qiang (2013-08-30). "Animal biodiversity: An update of classification and diversity in 2013. In: Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013)". Zootaxa. 3703 (1): 5. doi:10.11646/zootaxa.3703.1.3.

6. Hendricks, D. M. (n.d.). 5. Animals and Soil in Arizona. Retrieved March 06, 2018, from

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