The skeletal system is often seen as a singular form; a system that has no variation, nor functions other than movement. The truth is that the skeletal system takes many forms in various species and performs various functions. Although movement is an important function of the skeletal system, the other benefits and necessity of having a complete set of functioning bones cannot be ignored. One of the most common types of skeletons is the "exoskeleton," which is found on creatures that have an outer shell, or "armor." (Hoefnagels). In this structure, the animal has an internal muscular system that pulls on the skeleton, creating movement within the body. In all vertebrate species, the skeleton serves as a mold and model for the shape the body will take; There are various types of skeletons, which serve individual purposes. For example, the hydrostatic skeleton holds fluid (water) within flexible layers of tissue to form a more flexible body; jellyfish have the so-called "bell", which is a "...gelatinous substance forced between t...