Movement and Support: Skeletal Systems

As organisms evolved from the ancestral protists to the multicellular animals, body size increased dramatically. Systems involved in movement and support evolved simultaneously with the increase in body size.

Four cell types contribute to movement: 1) amoeboid cells, 2) flagellated cells, 3) cilicated cells, and 4) muscle cells. With respect to support, organisms have three kinds of skeletons: 1) fluid hydrostatic skeletons, 2) rigid exoskeletons, and 3) rigid endoskeletons. These skeletal systems also function in animal movement that requires muscles working in opposition (antagonism) to each other.

The Skeletal system of Invertebrates

Many invertebrates use their body fluids for internal support. For example, sea anemones and earthworms have a form of internal support called the hydrostatic skeleton.

Hydrostatic Skeletons
The hydrostatic skeleton is a core of liquid (water or a body fluid such as blood) surrounded by a tension-resistant sheath of longitudinal and /or circular muscles. It is similar to a water-filled balloon because the force exerted against the incompressible fluid in one region can be transmitted to other regions. Contracting muscles push against a hydrostatic skeleton, and the transmitted force generates body movements, as we will see in lab with the movement of a Hydra. Another example is the earthworm, Lumbricus terrestris. It contracts its longitudinal and circular muscles alternately creating a rhythm that moves the earthworm through the soil. In both of these examples, the hydrostatic skeleton keeps the body from collapsing when its muscles contract.

The invertebrate hydrostatic skeleton can take many forms and shapes, such as the gastrovascular cavity of acoelomates, a pseudocoelom in nematodes, a coelom in annelids, or a hemocoel in molluscs. Overall, the hydrostatic skeleton of invertebrates is an excellent example of adaptation of major body functions to this simple but efficient principle of hydrodynamics—use of the internal pressure of body fluids.

Exoskeletons
Rigid exoskeletons also have locomotor functions because they provide sites for muscle attachment and counterforces for muscle movements. Exoskeletons also support and protect the body, but these are secondary functions.

In arthropods, the epidermis of the body wall secretes a thick, hard cuticle that waterproofs the body. The cuticle also protects and supports the animal's soft internal organs. In crustaceans (e.g., crabs, lobsters, shrimp), the exoskeleton contains calcium carbonate crystals that make it hard and inflexible—except at the joints. Besides providing shieldlike protection from enemies and resistance to general wear and tear, the exoskeleton also prevents internal tissues from drying out. This important evolutionary adaptation contributed to arthropods' successful colonization of land. Exoskeletons, however, limit an animal's growth. Most animals shed the exoskeleton periodically, as arthropods do when they molt.

Certain regions of the arthropod body have a thin, flexible cuticle, and joints (articulations). It is in these areas that pairs of antagonistic muscles function through a system of levers to produce coordinated movement. Interestingly, some arhropod joints (e.g., the wing joints of flying beetles and the joints of fleas involved in jumping) have a highly elastic protein called resilin. Resilin stores energy on compression and then releases the energy to produce movement. From an evolutionary perspective, the development of a jointed, flexible exoskeleton that permitted flight is one of the reasons for the success of arthropods.

Endoskeletons
Like the term implies, other body tissues enclose endoskeletons. For example, the endoskeletons of sponges consist of mineral spicules or fibers of spongin that keep the body from collapsing. Since adult sponges attach to the substrate, they have no need for muscles attached to the endoskeleton. Similarly, the endoskeletons of echindoerms (sea stars, sea urchins) consist of small, calcareous plates called ossicles. Of course, the most familiar endoskeletons are in vertebrates.