Nervous System In Cnidarians

Nervous responses in cnidarians are controlled by a diffuse web of interconnected nerve cells called a nerve net. In many cnidarians, like the polyp shown in Figure 33-5, the nerve net is distributed uniformly throughout the entire body. There is no brain or similar structure that controls the rest of the nerve net. But in the medusa form of some cnidarians, such as jellyfish, some of the nerve cells are clustered in rings around the edge of the bell-shaped body.

The nerve net enables cnidarians to respond to specific stimuli in their environment. For example, when cells in the epidermis are touched, they relay a signal to nerve cells. The nerve cells, in turn, transmit a signal via the nerve net to contractile cells, which can cause the animal to withdraw from the stimulus. In cnidarians with the simplest nerve nets, a stimulus anywhere on the body causes signals to be sent through the nerve net in all directions. These signals bring about a contraction of the entire body.

figure 33-4

(a) The nematocyst inside this cnidocyte contains a coiled filament.

(b) When something touches the "trigger," the nematocyst suddenly ejects the filament.

figure 33-4

(a) The nematocyst inside this cnidocyte contains a coiled filament.

(b) When something touches the "trigger," the nematocyst suddenly ejects the filament.

figure 33-5

The interconnected nerve cells in the nerve net of this cnidarian (a hydra), coordinate the animal's responses to its environment.

The interconnected nerve cells in the nerve net of this cnidarian (a hydra), coordinate the animal's responses to its environment.

Quick Lab

Identifying Poriferans, Ctenophorans, and Cnidarians

Materials pencil, paper, a picture of either a poriferan, a ctenophoran, or a cnidarian

Procedure

1. Prepare a dichotomous key to differentiate among poriferans, ctenophorans, and cnidarians by writing paired statements for the three phyla and for classes Hydrozoa, Scyphozoa, and Anthozoa.

2. Statement (1a) describes cnidari-ans and leads to paired statements (3). Statement (1b) leads to paired statements (2).

3. Statement (2a) describes Porifera and (2b) describes Ctenophora.

4. Statement (3a) describes Hydrozoa and (3b) leads to paired statements (4).

5. Statement (4a) describes Scyphozoa and (4b) describes Anthozoa.

6. Get a picture from your teacher. Identify the animal using your dichotomous key. Exchange keys with another group, and use the new key to identify your animal.

Analysis What do sponges and hydrozoans have in common? How do comb jellies differ from cnidari-ans? What is the dominant body form in each cnidarian class?

The nerve net also coordinates the complex activities of the body that are necessary for feeding and traveling through the environment. The movements by which the tentacles bring prey to the mouth and push it into the gastrovascular cavity are controlled by the nerve net, as are the rhythmic contractions of the body that propel swimming medusae through the water.

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