The Hierarchy of Structure in Animals
Multicellular organisms, such as animals, are made up of many different specialized types of cells. Each cell is specialized to perform a particular
function. Less unusual cells, but still highly specialized, include muscle and bone cells, blood cells, and sensory cells responsible for detecting sights, sounds, and odours. Single-celled organisms, such as bacteria and blue-green algae, function independently. They do not directly depend on any other cells. In contrast, specialized animal cells cannot survive on their own. These cells live and work as part of a much larger group of cells that collectively make up the body of the animal. The complexity of animal bodies varies considerably. Some animals, such as sponges, have a simple body structure. Slugs and snails are more complex.
Multicellular organisms, such as animals, are made up of many different specialized types of cells. Each cell is specialized to perform a particular
function. Less unusual cells, but still highly specialized, include muscle and bone cells, blood cells, and sensory cells responsible for detecting sights, sounds, and odours. Single-celled organisms, such as bacteria and blue-green algae, function independently. They do not directly depend on any other cells. In contrast, specialized animal cells cannot survive on their own. These cells live and work as part of a much larger group of cells that collectively make up the body of the animal. The complexity of animal bodies varies considerably. Some animals, such as sponges, have a simple body structure. Slugs and snails are more complex.
All animals are made up of cells that are organized in a way that allows them to perform all of life’s functions. There are levels of organization within each animal. These levels of organization form a hierarchy, with the “most complex” at the top and the “least complex” at the bottom. Consider the hierarchy of organization within a specific animal: a white tailed deer. Let’s start with the simplest level of organization. Figure 2(a) shows a single muscle cell in the deer’s heart. Each heart muscle cell is branched, allowing it to connect to other heart muscle cells. Together, these
muscle cells make up the muscle tissue (Figure 2(b)). Figure 2(c) shows the heart itself, at the organ level of the hierarchy. An organ is made up of two
or more types of tissues that work together to perform a complex function. In addition to muscle tissue, the heart includes two other types of tissue:
nerve tissue and connective tissue. An organ system consists of one or more organs and other structures that work together to perform a vital body
function. The heart, blood vessels, and blood are all parts of the circulatory system (Figure 2(d)). The organism—in this case, the deer in Figure 2(e)--
is made up of many different organ systems working together.
muscle cells make up the muscle tissue (Figure 2(b)). Figure 2(c) shows the heart itself, at the organ level of the hierarchy. An organ is made up of two
or more types of tissues that work together to perform a complex function. In addition to muscle tissue, the heart includes two other types of tissue:
nerve tissue and connective tissue. An organ system consists of one or more organs and other structures that work together to perform a vital body
function. The heart, blood vessels, and blood are all parts of the circulatory system (Figure 2(d)). The organism—in this case, the deer in Figure 2(e)--
is made up of many different organ systems working together.
Organ Systems
All animals accomplish the same basic functions regardless of their appearance, behaviour, or where they live. They all obtain oxygen and nutrients and eliminate wastes. They all sense and respond to their environment, grow and repair damage, and reproduce. The task of organ systems is to perform these basic functions. Some well-known human organ systems are illustrated in the image below.
All animals accomplish the same basic functions regardless of their appearance, behaviour, or where they live. They all obtain oxygen and nutrients and eliminate wastes. They all sense and respond to their environment, grow and repair damage, and reproduce. The task of organ systems is to perform these basic functions. Some well-known human organ systems are illustrated in the image below.
Organs
Each organ system is made up of highly specialized organs and other structures that work together to perform the overall function of the system.
For example, the digestive system is made up of many organs, including the stomach, small and large intestines, liver, and pancreas. Most organs
work within a single organ system. For example, the stomach is part of the digestive system and of no other system. Some organs, however, play a role
in more than one system. For example, the pancreas is part of the digestive system and the endocrine system.
Each organ system is made up of highly specialized organs and other structures that work together to perform the overall function of the system.
For example, the digestive system is made up of many organs, including the stomach, small and large intestines, liver, and pancreas. Most organs
work within a single organ system. For example, the stomach is part of the digestive system and of no other system. Some organs, however, play a role
in more than one system. For example, the pancreas is part of the digestive system and the endocrine system.
Tissues
Animals have four major types of tissue: epithelial tissue, connective tissue, muscle tissue, and nerve tissue. Each of these types of tissue contains many
types of specialized cells, and each is found in most organ systems. Table 1, summarizes these four types of tissue.
Animals have four major types of tissue: epithelial tissue, connective tissue, muscle tissue, and nerve tissue. Each of these types of tissue contains many
types of specialized cells, and each is found in most organ systems. Table 1, summarizes these four types of tissue.