* Defining "Skin": Skin, as we typically understand it, is a protective outer layer made of tissue. It helps with things like:
* Protection from the environment: Against water, sunlight, injury, and infection.
* Regulation of temperature: Sweating or shivering to maintain body temperature.
* Sensory input: Touch, pressure, and temperature.
* Excretion: Some animals excrete waste products through their skin.
* Animals without "Skin": Some animals lack a true skin layer as described above, instead having other protective coverings:
* Insects: Have an exoskeleton made of chitin, a tough, flexible material.
* Jellyfish and other Cnidarians: Have a thin outer layer called an epidermis, but it's not as complex as vertebrate skin.
* Sponges: Lack a true skin and instead have a network of cells called a pinacoderm.
* Sea anemones: Have an outer layer called an ectoderm, which is more similar to a membrane than skin.
It's all about adaptation: The type of outer covering an animal has depends on its environment, lifestyle, and evolutionary history. All animals need some sort of protection, and "skin" is just one of many ways to achieve that.