Cnidarians, the phylum that includes familiar organisms like jellyfish, sea anemones, and corals, utilize a form of muscle to achieve movement. These aquatic invertebrates rely on contractile tissues to change shape, retract, and propel themselves through the water. This contractile tissue is fundamentally different from the true, dedicated muscle found in more complex animals such as vertebrates and arthropods. Instead of having a third distinct tissue layer for muscle development, Cnidarians employ specialized cells that perform dual roles. This unique biological adaptation allows them to execute a range of movements necessary for feeding, defense, and locomotion.
The Specialized Contractile Cells
Cnidarians are considered diploblastic, meaning their body wall is derived from only two primary tissue layers: the outer epidermis and the inner gastrodermis. The contractile tissue in these organisms is made up of unique structures called epitheliomuscular cells. These cells combine the function of an epithelial cell, which forms a protective covering, with the ability to contract, characteristic of muscle cells.
The base of each epitheliomuscular cell extends into a long, thin fiber containing myofilaments, the protein structures responsible for contraction. In the outer epidermis, these fibers typically run lengthwise along the body, forming a layer of longitudinal contractile elements. Conversely, the epitheliomuscular cells in the inner gastrodermis often have fibers arranged in a circular pattern around the body wall. This arrangement of opposing fiber sheets allows a Cnidarian to change its dimensions and shape dramatically.
Coordinating Contraction and Movement
Movement in Cnidarians is coordinated by a decentralized nervous system known as a nerve net. This diffuse network of interconnected nerve cells is spread throughout the body wall, allowing impulses to be transmitted in multiple directions without a centralized brain. When a stimulus is received, the nerve net rapidly conducts the signal, resulting in a coordinated contraction across a sheet of epitheliomuscular cells.
This muscular action functions against a physical framework called a hydrostatic skeleton. The central gastrovascular cavity is filled with water, and when the animal closes its mouth, this water is trapped. The pressure of the incompressible water in the cavity provides a rigid structure against which the contractile cells can push. A contraction of the circular fibers, for example, increases the internal pressure, causing the animal to lengthen and become thinner.
Movement in Different Cnidarian Body Forms
The two main body plans of the phylum, the sessile polyp and the free-swimming medusa, use the same basic contractile system to achieve different movements. The medusa form, commonly known as a jellyfish, uses rhythmic, coordinated contractions of the bell margin to propel itself through the water. This action forces water out from under the bell, generating a pulse of movement known as jet propulsion. The speed of these contractions can be adjusted, allowing the jellyfish to drift passively or move with intent toward the water surface or away from a threat.
The polyp form, such as a sea anemone or Hydra, uses its contractile fibers for postural adjustments, defense, and slow relocation. Rapid contraction of the longitudinal fibers allows a polyp to quickly shorten and pull its body and tentacles into a small, protective ball. Some polyps, like sea anemones, can slowly glide along the substrate on their pedal disc. Others, like Hydra, can move by somersaulting or inching along the surface.