I am afraid worms are not the most pleasant topic to write about but few people know much about them. I was once told when I was a student that if you wanted to become known as a scientist study worms, no one else is.
When we hear the term “worm” negative things enter our minds: parasites, disease, uncleanliness to name a few, but many worms are actually beneficial by removing detritus (decaying organic matter) from the environment; the garbage cleaners in a sense. There are at least 10 phyla of worms but this series will focus on the three major groups; flatworms, roundworms, and segmented worms.
Flatworms include three classes and two of those are parasitic; those are the flukes and tapeworms. Most are very small and emerge in low or no light. The parasitic forms typically live in the gut but can infest other organs of their host organism. There are several species that infest humans but most are specific to a particular group of animals. The flatness of their bodies may have to do with moving materials in and out of the body. Most flatworms lack well develop organ systems so gas exchange occurs through the skin. The more the flat they are, the more surface area they have, the more gas exchange can occur. This is supported by the fact that the larger the flatworm is the more flat they are.
Tubellarians are basically non-parasitic flatworms and are mostly aquatic, many living in the marine environment. Some crawl across the seabed but others can actually swim. As with other flatworms, their digestive tract is incomplete (meaning there is only one opening – the mouth – where food comes in and waste goes out), and this mouth is located half way down their body on the ventral side. Most of these flatworms are carnivorous feeding on small invertebrates and dead organisms. They do have “eyespots” which do not form images but can detect light. Most flatworms are what we call “negatively phototaxic” meaning they sense light but do not like it and will burrow or hide when the sun rises.
Trematoda are what we call flukes and are parasitic. Most are only a few centimeters long but some can reach a meter (3ft.) or more! Flukes have a protective covering on their skin to protect them from the enzymes of their host’s internal environment. Their life cycle requires a second host, meaning that the adult lives in one type of animal but the larval stage occurs in another. Adult flukes live in vertebrates (typically fish), and the secondary hosts are usually invertebrates (typically snails). The eggs (cyst) produced by the adults leave the host organism through their feces. Once in the environment the secondary host consumes them where the larva develop. Eventually the secondary host is consumed by the primary host (fish) where the larva develop into an adult and the cycle begins again. They typically infest the gut but can infest other organs as well.
Cestods are one of the more recognized flatworms; these are the tapeworms. Tapeworms lack a digestive tract and most absorb all of their nutrients on through their flat bodies. Like their fluke cousins, tapeworms are endoparasites but almost all of them infest the digestive tract. Like their fluke cousins they require a secondary host, usually an arthropod (insect, spider, or crustacean). With a vertebrate serving as the host organism.
Though there are flukes and tapeworms that infest humans most are found in fish and are specific to that group. The ones that do infest humans require the secondary host cycle described above and, because of sanitary conditions we live in, are not commonly found in the population. This cannot be said for parts of the world where sanitary conditions are not to our standards. As horrible as parasites sound many species of nonparasitic flatworms are beneficial by removing detritus from lakes, rivers, and bays.
Next week… Roundworms.