Invertebrate Phyla Other Than Arthropods 159

lengths. They have no digestive system, but absorb food from the host gut through tiny projections similar to the villi of the intestines themselves. They shed reproductive organs from their end, which leave with the feces or crawl out the anus, spreading larvae for subsequent infections. Pork tapeworm can migrate to the eye or brain of humans, causing blindness or death. About 1% of American cattle are infected, and 20% of slaughtered cattle are not federally inspected. Furthermore, inspections miss 25% of infections. These facts point up the importance of cooking meat thoroughly, which kills tapeworms.

The nine phyla of the vermiform (worm-shaped) group have a complete mouth-to-anus digestive system. Phylum rotifera (the rotifers, Figure 8.2) have one or more rings of cilia at the head to draw food toward the mouth. When the cilia are beating, they give the impression of a rotating wheel. The foot has one to four toes that it uses for attachment. There are about 1800 species, all of which are dioecious. Most live in benthic aquatic habitats. They can withstand desiccation for long periods.

The phylum nematoda contains the nematodes, or roundworms. Nematodes are among the most abundant animals on Earth. There can be 3 billion per cubic meter of soil, where they help to recycle organic matter. They are common parasites in pets and other animals. Ascaris lumbricoides is a large roundworm that commonly infects humans. A female may lay 200,000 eggs a day, which pass in the feces. Trichinella spiralis causes the disease trichinosis. In the past this disease was contracted by eating poorly cooked pork, but it has been virtually eliminated in the United States. T. spiralis is actually an intracellular parasite that can control the gene expression of its host cell. Eight species of filarial

Figure 8.2 The rotifer Philodina, showing internal structure. (From Hickman et al., 1997. © The McGraw-Hill Companies. Used with permission.)

worms infect humans, including one that causes the disease called river blindness, which is carried by black flies and is common in hot climates.

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