By the end of this module,
Animals are heterotrophs, meaning they cannot fix carbon, and require organic carbon compounds for growth. By consuming reduced carbon compounds, heterotrophs can use all the energy they obtain from food in their growth and reproduction (unlike autotrophs that must use some of their energy for carbon fixation). However, heterotrophs are unable to make their food, and can die from an absence of food. All heterotrophs feed directly or indirectly on other living things. Unlike animals, fungi are heterotrophs that digest food outside of their body before taking up the nutrients through absorption, whereas animals ingest their food and then digest it internally
Animals are multicellular organisms. Their cells are distinguished from those of other eukaryotes, most notably plant cells, by their lack of cell walls and chloroplasts. Due to the lack of a rigid cell wall, animal cells can adopt a variety of shapes. A phagocytic cell can even engulf other structures. Because they lack cell wall support, animals have extracellular proteins, the most abundant being collagen.
Animals have many cell types that are not found in other multicellular organisms, including muscle cells and nerve cells. They also have unique tissue types, including connective, muscle, nervous, and epithelial tissues.
Most animals develop from three primary germ layers (termed triploblastic), with lower forms developing from two (diploblastic). Animal cells lack cell walls, and their bodies exhibit some form of body symmetry:
Figure 1. Animals exhibit different types of body symmetry. The (a) sponge, shown in a peach color, is asymmetrical and has no planes of symmetry, the (b) sea anemone, shown in white, has radial symmetry with multiple planes of symmetry which are shown in blue, and the (c) browngoat has bilateral symmetry with one plane of symmetry, also shown in blue.
"This work" by , OpenStax College is licensed under CC BY 3.0
Animals are also classified according to the type of body cavity they possess (body cavity - a space between the digestive cavity and body wall) and also according to embryological development (protostomes and deuterostomes). What type of digestive tract is another characteristic observed in animals:
• Incomplete: have only one opening which serves as both mouth and anus
• Complete: have both a mouth and an anus
Cephalization or the presence of a head and localization of sensory organs is also an important characteristic to observe. Most animals are divided into two groups, invertebrates, and vertebrates. Most animals (97%) are invertebrates and are termed such as they lack a backbone.
Date last modified: August 30, 2016.
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