Anatomy and Physiology
Figure 1. General external anatomy of a sea anemone. Adapted from Ruppert, Fox & Barnes (2004).
Figure 1 shows the general anatomy of a sea anemone. The body is made up of a tubular column with an oral disc sitting at the top and the base expanding into a pedal disc1 that can adhere to substrate reversibly, possibly by a protein-chitin cement2. The mouth occurs in the middle of the oral disc and tentacles are situated along the border. Beneath the oral disc, a tubular and laterally compressed actinopharynx (or pharynx) connects the mouth to the coelenteron, a body cavity, via an interal opening1. The actinopharynx also functions as a valve to retain seawater in the coelenteron3. At the two opposite corners of the mouth are the siphonoglyphs, which are longitudinal grooves at the end(s) of the actinopharynx equipped with ciliation to carry water into the coelenteron. This functions to maintain the volume of coelenteric fluid and supply oxygenated water to the mesenteries in sea anemones, particularly for larger individuals where gas diffusion becomes limited4.
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| Figure 2. Transverse-section of sea anemone through actinopharynx region. Adapted from Hutchings, Kingsford & Hoegh-Guldberg (2008) and Ruppert, Fox & Barnes (2004). |
Vertical partitions known as mesenteries (or septa) divide the coelenteron into radial compartments. Each mesentery is an outfold of gastrodermis and mesoglea, and occurs as a layer of mesoglea between two layers of gastrodermis. If the mesenteries run the entire length of the coelenteron and join the pharynx, they are known as complete mesenteries, if they do not, they are incomplete mesenteries (Figure 2). They may also insert on the pedal and oral discs1, and may be seen on the external surface of the oral disc as thin lines or stripes4. Tentacles arise between a pair of mesenteries as a holllow outgrowth of the body wall1. Digestion and absorption and gamete development occur at the mesenteries, which also bear the major longitudinal musculature important for defense purposes. The mesenteries are added throughout the life of an individual as it grows, which may confer increased mechanical, nutritional and energetic advantages4.
The inner margin of each mesentery is swollen and may be partitioned into either one or three lobes. If there are three lobes, the middle lobe is known as the cnidoglandular band and has cnidocytes (see Defense Mechanisms) and enzymatic gland cells (see Digestive System). The other two lateral lobs are known as the flagellar bands and are densely flagellated, with flagella that beat toward the central axis of the body (see Digestive System) (Figure 3). If there is only one lobe, the middle area will contain the cnidoglandular cells while flagella will be situated along the lateral regions1.
Figure 3. A: Transverse section through a mesentery in a sea anemone. B: Transverse magnified section of upper, trilobed region of mesentery. C: Transverse magnified section of lower, unilobed region of mesentery. Arrows indicate direction of ciliary flows of fluid and food. Adapted from Ruppert, Fox & Barnes (2004).
Animals in the phylum Cnidaria have radial symmetry as a result of the polyp’s sessile lifestyle, where it faces the environment in all directions. The class Anthozoa however, is actually biradially symmetrical along a directive axis which is defined by a plane that longitudinally bisects the mouth, actinopharynx, siphonoglyph and directive pairs of mesenteries. The mesenteries in Anthozoans are present on opposite sides of this axis as mirror images (Figure 2). This arrangement can be attributed to their hydrostatics2 and their size and ventilation requirements for the coelenteron5.
In Cryptodendrum adhaesivum, the sphincter is noted to be circumscribed and weak. There are numerous mesenteries, with more occurring at the margins of the animal than at the base6.
1Ruppert, Fox & Barnes 2004
2Shelton 1982
3Pantin 1960
4Shick 1991
5Hyman 1940
6Dunn 1981
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