Phylum Ctenophora

The phylum Ctenophora includes about 100 species of marine animals known as ctenophores. A typical ctenophore is shown in Figure 33-12. Ctenophora means "comb holder" and refers to the eight comblike rows of cilia that run along the outside of the animal. Ctenophores resemble jellyfish and are often called comb jellies.

Ctenophores differ from jellyfish and other cnidarians in several ways. Rather than pulsating like jellyfish, they move through the water by beating their cilia. They are the largest organisms to move in this fashion. Also, ctenophores do not have cnidocytes. Instead, many have cells called colloblasts, which secrete a sticky substance that binds to their prey. Colloblasts are usually located on two tentacles. Ctenophores also have a sensory structure called an apical organ at one end of their body. This organ enables ctenophores to sense their orientation in the water. Nerves running from the apical organ coordinate the beating of the cilia. Most ctenophores are hermaphroditic.

One of the most striking features of ctenophores is their bioluminescence (BlE-oh-loo-muh-NES-ens), or production of light by means of a chemical reaction. Bioluminescent ctenophores often occur in large swarms near the surface of the ocean, which creates a spectacular display at night.

figure 33-12

A ctenophore, Pleurobrachia pileus, moves through the water by beating its cilia, trailing its two long tentacles behind it.

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