28.2  Phylum Cnidaria

Mary Ann Clark; Matthew Douglas; Jung Choi

28.2 Phylum Cnidaria

Learning Outcomes

Compare structural and organization characteristics Cnidaria
Describe the progressive development of tissues and their relevance to animal complexity
Identify the two general body forms found in the Cnidaria
State the major cnidarian classes

Phylum Cnidaria includes animals that exhibit radial or biradial symmetry and are diploblastic, meaning that they develop from two embryonic layers, ectoderm and endoderm. Nearly all (about 99 percent) cnidarians are marine species.

The defining cell type for the cnidarians is the cnidocyte, or stinging cell. These cells are located around the mouth and on the tentacles, and serve to capture prey or repel predators. Cnidocytes have large stinging organelles called nematocysts, which usually contain barbs at the base of a long coiled thread.

The illustration shows a nematocyst before, shown as a; and after, shown as b, firing. The nematocyst is a large, oval organelle inside a rectangular cnidocyte cell. The nematocyst is flush with the plasma membrane, and a touch-sensitive hairlike projection extends from the nematocyst to the cell's exterior. Inside the nematocyst, a thread is coiled around an inverted barb. Upon firing, a lid on the nematocyst opens. The barb pops out of the cell and the thread uncoils. — **Figure 28.5** Cnidocytes. Animals from the phylum Cnidaria have stinging cells called cnidocytes. Cnidocytes contain large organelles called (a) nematocysts that store a coiled thread and barb, the nematocyst. When the hairlike cnidocil on the cell surface is touched, even lightly, (b) the thread, barb, and a toxin are fired from the organelle.

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Morphology of Cnidarians

Two distinct body plans are found in Cnidarians: the polyp or tuliplike “stalk” form and the medusa or “bell” form. (Figure 28.6). An example of the polyp form is found in the genus Hydra, whereas the most typical form of medusa is found in the group called the “sea jellies” (jellyfish). Polyp forms are sessile as adults, with a single opening (the mouth/anus) to the digestive cavity facing up with tentacles surrounding it. Medusa forms are motile, with the mouth and tentacles hanging down from an umbrella-shaped bell.

The illustration compares the medusa (a) and polyp (b) body plans. The medusa is dome-shaped, with tentacle-like appendages hanging down from the edges of the dome. The polyp looks like a tree, with a trunk at the bottom and branches at the top. Both the medusa and polyp have two tissue layers, with mesoglea in between. The mesoglea is thicker in the dome of the medusa than in the polyp. Both also have a central body cavity. — **Figure 28.6** Cnidarian body forms. Cnidarians have two distinct body plans, the medusa (a) and the polyp (b). All cnidarians have two membrane layers, with a jelly-like mesoglea between them.

All cnidarians are diploblastic and thus have two “epithelial” layers in the body that are derived from the endoderm and ectoderm of the embryo. The outer layer (from ectoderm) is called the epidermis and lines the outside of the animal, whereas the inner layer (from endoderm) is called the gastrodermis and lines the digestive cavity. In the planula larva, a layer of ectoderm surrounds a solid mass of endoderm, but as the polyp develops, the digestive or gastrovascular cavity opens within the endoderm. A non-living, jelly-like mesoglea lies between these two epithelial layers. In terms of cellular complexity, cnidarians show the presence of differentiated cell types in each tissue layer, such as nerve cells, contractile epithelial cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as the presence of intercellular connections. However, the development of organs or organ systems is not advanced in this phylum.

Physiological Processes of Cnidarians

The nervous system is rudimentary, with nerve cells organized in a network scattered across the body.

The gastrovascular cavity has only one opening that serves as both a mouth and an anus; this arrangement is called an incomplete digestive system. In the gastrovascular cavity, extracellular digestion occurs as food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb nutrients. However, some intracellular digestion also occurs. The gastrovascular cavity distributes nutrients throughout the body of the animal, with nutrients passing from the digestive cavity across the mesoglea to the epidermal cells. Thus, this cavity serves both digestive and circulatory functions.

Cnidarian cells exchange oxygen and carbon dioxide by diffusion between cells in the epidermis and water in the environment, and between cells in the gastrodermis and water in the gastrovascular cavity. The lack of a circulatory system to move dissolved gases limits the thickness of the body wall and necessitates a non-living mesoglea between the layers. In the cnidarians with a thicker mesoglea, a number of canals help to distribute both nutrients and gases. There is neither an excretory system nor organs, and nitrogenous wastes simply diffuse from the cells into the water outside the animal or into the gastrovascular cavity.

The phylum Cnidaria contains about 10,000 described species divided into two monophyletic clades: the Anthozoa and the Medusozoa. The Anthozoa include the corals, sea fans, sea whips, and the sea anemones. The Medusozoa include several classes of Cnidaria in two clades: The Hydrozoa include sessile forms, some medusoid forms, and swimming colonial forms like the Portuguese man-of-war. The other clade contains various types of jellies including both Scyphozoa (jellyfish) and Cubozoa (box jellies) The Anthozoa contain only sessile polyp forms, while the Medusozoa include species with both polyp and medusa forms in their life cycle.