Muscles are responsible for movement in animals. If there were no muscular tissue in an animal body, the following would happen:

• No movement: Without muscles, the animal would not be able to move at all.

• No shape: Muscles provide structure and shape to the body. Without them, the body would have no shape and would be like jelly.

• No regeneration: Some animals, such as C. elegans, lack satellite cells (muscle stem cells) and therefore muscles cannot regenerate.

However, some animal groups dont have any muscles at all, as they branched off from the evolutionary path before muscle cells evolved. Yet these animal groups are not incapable of movement. For instance, sea sponges are able to contract without muscles. Scientists have identified that the sponge epithelial cells are actually contracting, which offers new approaches to understanding the evolutionary development of musculature.