**Cellular Level of Organization in Animalia**

The cellular level of organization is the basic level of biological organization in animals. It refers to the arrangement of cells and their specific functions within an organism. At this level, cells are the building blocks of the animal body, and they work together to maintain the overall function and homeostasis of the organism. Let's take a closer look at the cellular level of organization in Animalia.

**Cell Structure and Function:**
- Animal cells are eukaryotic cells, meaning they have a distinct nucleus and membrane-bound organelles.
- Cells perform various functions based on their specialized structures and organelles.
- For example, muscle cells contain contractile proteins that enable movement, nerve cells have long extensions called axons for transmitting electrical signals, and epithelial cells form barriers and linings in the body.

**Tissues:**
- Tissues are groups of similar cells that work together to perform a specific function.
- In animalia, there are four primary types of tissues: epithelial, connective, muscular, and nervous.
- Epithelial tissues line body surfaces and cavities, providing protection and secretion.
- Connective tissues support and connect different body parts, such as bones, tendons, and blood.
- Muscular tissues generate force and movement, allowing animals to move and perform various actions.
- Nervous tissues enable communication and coordination within the body through the transmission of electrical signals.

**Organs:**
- Organs are composed of different tissues working together to perform specific functions.
- For example, the heart is an organ made up of cardiac muscle tissue, connective tissue, and nervous tissue.
- The heart's cardiac muscle tissue contracts to pump blood, connective tissue provides support and structure, and nervous tissue coordinates the heart's rhythm.

**Organ Systems:**
- Organ systems are groups of organs that work together to perform a particular function.
- In animals, there are various organ systems such as the circulatory, respiratory, digestive, nervous, and reproductive systems.
- Each organ system has specific organs that work together to carry out functions essential for the survival and maintenance of the organism.

**Integration and Homeostasis:**
- At the cellular level of organization, cells communicate and interact with each other to maintain homeostasis.
- Homeostasis refers to the stable internal environment required for the proper functioning of cells and the organism as a whole.
- Cells receive signals from their environment and other cells to adjust their activities and maintain balance.
- For example, cells in the pancreas sense changes in blood glucose levels and release insulin or glucagon to regulate blood sugar levels.

In conclusion, the cellular level of organization in Animalia involves the arrangement of cells, tissues, organs, and organ systems. Cells perform specialized functions within tissues, which then work together to form organs. These organs further integrate to form organ systems, allowing animals to carry out various vital functions and maintain homeostasis.