Fabulous flatworms

The phylum Platyhelminthes is so big (a beefy 34,000 species) and so diverse that a single lecture or chapter in an invertebrate zoology text book just can't do them justice. So here's a bit more. First of all, here's a gorgeous, free-living Hawaiian flatworm. Even large flatworms like these typically crawl along the bottom, relying on primarily on ciliary gliding. However, they are also able to swim, relying on exquisite coordination of their muscle layers to generate graceful motion that propels them through the water.

And then there are those parasitic flatworms. Definitely read about the trematode life cycles that can be found in every invertebrate zoology textbook on the shelf: Chinese liver fluke, blood fluke (the cause of schistosomiasis), and the lancet liver fluke, which infects sheep and other ruminants. In this fluke, one of the intermediate hosts, an ant, has its brains addled by the parasite, causing it to crawl to the top of a grass blade, bite down and hang on. This behavior makes it more likely for the infected ant to be eaten by the final host, a sheep, in which the parasite can complete its life cycle.
I also promised my students more information on other trematodes not mentioned in the text book. The lung fluke, Paragonimus westermani, relies on humans and felines as the definitive host. The adult worms live in small capsules embedded deep in the tissue of the host's lungs. The sexually mature worms produce eggs, which are shed through the host's mouth as they are coughed up, or with the host's feces if they are swallowed by the host after being coughed up. Once released into fresh water, the larva hatches and swims to its first intermediate host, a snail. Several cycles of asexual reproduction occur within the snail (sporocyst produces many rediae; each redia produces more rediae or many cercariae). The cercariae leave the snail and penetrate a crab where they encyst in the muscle. To reach sexual maturity, the immature flukes must be ingested by the final host, which preys on the infected crab. If the crab is not thoroughly cooked, and cats never cook their food adequately to kill parasites, the fluke larvae move through the definitive host's gut wall and burrow into the lungs, where they reach sexual maturity. And one last cool micro-fact: this disease was first describe in two Bengal tigers that died in European zoos in 1878, so says Wikipedia.
And if the flukes aren't alarming enough, there are the tapeworms, another class of flatworms, with 3400 species. These are gut parasites of all major vertebrate taxa, and also typically rely on vertebrate or arthropod intermediate hosts. In contrast with the fluke life cycles, the larval stages of the tapeworms are usually passive, and transmission to the next host relies on the intermediate host being eaten along with its tapeworm parasites. In the tapeworm that affects your dog, eggs are shed via the anus. The eggs are eaten by larval fleas and as the fleas mature, they are swallowed inadvertantly by the dog in its typically routine of licking, nibbling, and grooming. Once in the dog's gut, the tapeworms grow and produce copious eggs. When the eggs leave the dog, they are released in packets, tiny wriggling bits of the tapeworm itself, called proglottids. Dogs that scoot their bottoms along the floor are responding to the sensation of these eggs packets leaving their bodies.
In the pork tapeworm, Taenia solium, humans are the only definitive host. Eggs are shed with the host's feces and must be ingested by an intermediate host to continue the life cycle. The intermediate host is usually a pig, and the larvae encyst in the pig's muscle. Eating undercooked pork results in the cysts developing into adult worms in the intestines of its host. These worms can be several meters long and produce eggs for many years. This may sound plenty gross, but an even worse problem results if a human rather than a pig serves as an intermediate host by ingesting eggs. If a person infected with an adult tapeworm prepares food without adequately washing his hands, eggs can be ingested by anyone eating that food. Also, autoinfection is possible if someone with a tapeworm infection vomits. I leave the details for the reader to discern. In any case, the eggs also make cysts in the human as an intermediate host, but they are typically found in the lung, liver, eye, or brain, and have severe consequences.
That's enough revolting parasite stories for now. Don't worry, we'll get to nematodes shortly. In the meantime, you can always check out the latest from Carl Zimmer.