slide 1

Let's begin with the type of skeleton that is probably the least familiar to us. Most cnidarians and worms have a hydrostatic skeleton made up of fluid under pressure in closed compartments. Animals with hydrostatic skeletons can change their form and move by changing the shape of the compartments.

slide 2

Because water can't be compressed very much, the volume of each compartment is relatively constant. In general, each compartment has circular muscles surrounding it, and longitudinal muscles running lengthwise along the compartment. When the diameter of a compartment is decreased by the contraction of the circular muscles, the length of the compartment must increase. Similarly, when the length of the compartment is decreased by contraction of the longitudinal muscles, the diameter of the compartment increases. This is how a sea anemone can display two very different forms.

slide 3

Earthworms and most other annelids use the ability to change the shape of the compartments in order to move. Each segment of an earthworm is a separate compartment. Earthworms move by peristalsis, a type of movement produced by rhythmic waves of muscle contractions from head to tail.

Starting at the head, the circular muscles contract to lengthen the compartments, moving the head segments forward. As the longitudinal muscles contract, the head compartments become shorter, but bristles on the surface keep them from slipping backward. The head is now further forward than it used to be. The compartments behind the head lengthen. The rhythmic contractions move the worm forward along the ground.

slide 4

A hydrostatic skeleton is well suited for some aquatic organisms, and for land animals that crawl or burrow. But for terrestrial animals, a hydrostatic skeleton can't keep parts of the body off the ground very effectively. Since many land animals move by lifting parts of their bodies off the ground, they need a more rigid support system. Some aquatic organisms also need a more rigid support system

slide 5

Some organisms that require a solid skeleton have an exoskeleton, a hard casing on the surface of their body. A mollusk's exoskeleton is a shell composed mainly of calcium carbonate secreted from the mantle. For bivalves, like clams, the hinged shell can be closed by muscles attached to the inside of the shell.

Most arthropods, like this grasshopper, have a jointed exoskeleton called a cuticle. The cuticle is composed of nonliving material, including the polysaccharide chitin, and is secreted by the epidermis. Muscles that attach to the knobs and plates of the exoskeleton extend into the interior of the arthropod.

slide 6

Let's look at how muscles are used to move parts of an animal with an exoskeleton. The action of a muscle is always a contraction. The extension or relaxation of a muscle is passive, not active. This diagram of a grasshopper's leg shows the two types of muscles that enable the leg to move. An extensor muscle is responsible for extending or straightening the leg. A flexor muscle is responsible for bending the leg. Click on each muscle to make it contract and move the leg.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education