Skeletons and coeloms

Abstract Skeletons are structural elements that provide support, muscle anchoring points, and protection. Skeletons can be external or internal. External skeletons are composed of plates and offer physical protection. Internal skeletons are often composed of bones. For a rigid mineralized skeleton to allow movement, there have to be joints between individual elements. The type of movement and locomotion an animal can carry out depends on what type of skeleton it has, if any. The main muscles used for locomotion tend to be body wall muscles, which can be longitudinal or circular. Coordinated activity of body wall muscles can lead to peristaltic or sinusoidal types of locomotion. Animals with appendages display other types of locomotion, such as walking or swimming. All muscle activity must be done against some sort of support structure. Animals without rigid mineralized skeletons often have a hydrostatic skeleton or coelom, which serves as the semirigid anchoring point for locomotory muscles.