Which of the following describes a hydrostatic skeleton?

The description of a hydrostatic skeleton refers to a structural support system that relies on fluid contained within a body cavity. In organisms with a hydrostatic skeleton, the fluid under pressure provides rigidity and enables the organism to maintain its shape. This system is particularly common in soft-bodied animals such as cnidarians (like jellyfish) and annelids (like earthworms).

The pressure exerted by the fluid allows these organisms to move effectively by contracting muscles around the body cavity, facilitating movement through expansion and contraction. This method of support is characterized by flexibility and adaptability, enabling a range of movements not feasible with rigid structures such as bone or cartilage.

Other options, such as a hard outer shell or a skeleton made of bone, do not fit the definition of a hydrostatic skeleton since they rely on solid structures for support and protection rather than fluid dynamics. Similarly, a cartilaginous skeleton, like that of sharks and rays, also does not involve fluid pressure but rather involves a more rigid form of structural support. Thus, the core characteristic of a hydrostatic skeleton is its reliance on pressurized fluid, which distinguishes it from other types of skeletal systems.