The sea anemone: members of Cnidaria
(Rust, fig. 53-56; C & V fig. 7.25-7.26)

Sea anemones belong to the Phylum Cnidaria, which includes jellyfish and corals. In an earlier lab you learned that Cnidaria have two body forms, medusa and polyp. In fact, three Classes of cnidarians are recognized based on body plan. Sea anemones and corals are placed in the Class Anthozoa, characterized by the lack of a medusa stage. Class Scyphozoa, including the jellyfish, lack a polyp stage, while Class Hydrozoa (hydras) have both medusa and polyp stages.

(A) Anatomy

Structurally the sea anemone is known as a polyp. The body wall consists of an outer layer of epidermis and an inner layer of gastrodermis; between these two is a gelatinous, non-cellular layer known as the mesoglea. The body wall is organized into a muscular column with a top that has a mouth (the oral disc) and a bottom (the pedal disc) that holds onto the substratum. Within the column is a sac-like digestive cavity, called the coelenteron or gastro-vascular cavity, that has the mouth as its only opening. The coelenteron of sea anemones is divided into pie-shaped sections by muscular mesenteries, some of which attach both to the top (oral) and bottom (pedal) discs. There is no right or left side on a sea anemone; it has radial symmetry.

Obtain an expanded sea anemone in a small dish of sea water. Locate the basic structures of the polyp and observe its reaction to mild stimuli such as touch. Place inanimate materials such as aluminum foil on the oral disc and watch its reaction. Then try a piece of liver or fish and watch the anemone's behavior. Finally gently prod the anemone with a blunt instrument so that it retracts its tentacles and pulls down against the bottom.

(B) A simple hydrostatic skeleton in action

How does an anemone, with no hard skeleton, stand up? Essentially it blows itself up with water. Ciliated channels called siphonoglyphs are located at one or both ends of the mouth. These continue on to the pharynx and pump water into the bag-like coelenteron. Continuous beating of the cilia pump sufficient water into the anemone to inflate the column and the hollow tentacles that are continuous with the coelenteron. The mouth closes and the cilia continue to beat to maintain a positive pressure within the coelenteron and provide sea water as the fluid for the hydrostatic skeleton. Muscle fibers in the column wall are organized in a circular configuration around the circumference of the column. Contraction of these muscles causes the column to decrease in diameter. As the column decreases in diameter, the water trapped in the coelenteron, since it is not compressible, forces the column to elongate. Elongation of the column causes the muscle fibers of the mesenteries, which are arranged parallel to the long axis of the column, to be stretched. When these longitudinal muscles contract the column shortens, the circular muscles in the column wall are stretched, and the diameter of the column increases. In response to vigorous stimuli the longitudinal muscles contract and the mouth opens, forcing the water out of the coelenteron and flattening the sea anemone against the bottom.

The sea anemone serves as a simple model of a hydrostatic skeleton. This skeleton involves the enclosure of a non-compressible fluid in a cavity between two sets of muscles that work against one another (called antagonistic muscles). Remember, muscles can only contract and relax, they can not stretch themselves. Compression of the enclosed fluid, be it sea water, body fluid or blood, by the contraction of one set of the antagonistic muscles causes the stretching of the other set of the antagonistic pair. Thus the hydrostatic fluid allows the animal's body to achieve a degree of rigidity sufficient to allow the muscles to work against one another and is identified as a hydrostatic skeleton. In the sea anemone movement is achieved even though it stands in place. The anemone is able to extend its column and tentacles, carry captured food to its mouth, pull in its tentacles, and flatten itself when in danger.