Coral (cnidaria) form colonies, building huge structures as their calcium carbonate skeletons add to others below. This accretion eventually builds into reefs and atolls. Coral reefs have bright colors due to symbiotic algae that live inside the coral cells. In recent years many reefs have been losing their algal symbionts, leading to the death of the reef. The cause of this coral bleaching is unknown, but it is feared that pollution and global warming are involved.
The three phyla in the group called flatworms show more organ specialization, including a primitive nervous system. Some have a gut with only a single opening rather than a separate mouth and anus. Others lack a gut and absorb food directly through their surface.
The phylum Platyhelminthes comprises the flatworms and flukes. There are four classes, of which we will describe three. The turbellarians include the small aquatic flat-worm Planaria (Figure 8.1), found on the bottom of rocks in streams. The trematodes include the blood fluke Schistosomona (also called Bilharzia), which causes the important waterborne parasitic infection schistosomiasis (or bilharziasis). This disease is common in warm climates. The schistosomes are discharged from the intestines of infected people in their feces. If the fecal contamination reaches a stream, the fluke can infect a specific kind of snail, where it develops. Released back to the water, the schistosomes can reinfect humans through the bare skin. It then travels via the bloodstream to infection sites in the liver and digestive system. Symptoms in humans include extreme diarrhea and bloody stool or urine. It can be controlled by proper sanitation and by control of conditions favorable to the snail host.
Tapeworms are in another class, the cestodes. Tapeworms are long (up to 10 m), very flat worms that infect vertebrate digestive tracts. Humans can contract them from improperly cooked meats. When ingested, they attach inside the intestines and grow to great
Figure 8.1 Planarian, showing internal structure. (From Hickman et al., 1997. © The McGraw-Hill Companies. Used with permission.)
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