As described, the life cycle pattern of the digenetic trematodes is quite complex, involving first- and second-stage larvae as well as adults that are dependent on a host species for survival. Upon hatching, the first-stage larvae, known as miracidia, infect the first intermediate host, usually a mollusk. Many miracidia take advantage of light and gravitational cues to reach an area suitable to their hosts, then hone in on an individual host by following its chemical signature, which may be fatty acids or amino acids specific to the organism. Other miracidia, however, seem to stumble upon rather than track their host. In still other trematode species, the egg does not develop until it is eaten by the host species.

Once in the first intermediate host, the miracidium sheds the ciliated epidermal cells that the aquatic forms typically use to navigate the water. In the first intermediate host, the miracidia travel to a specific site, depending on the species of the trematode and of the host. Once there, the miracidium may develop into a saclike sporocyst and/or redia before generating cercariae. Cercariae are the larvae that finally exit from the mollusk and begin actively seeking the second intermediate host. Cercariae are generally propelled by a variety of different types of tails, although a few species, like Maritrema arenaria, have small or no tails and move by crawling rather than swimming. Cercariae use environmental cues, like light or water turbulence (in aquatic species), to seek out their secondary host. A few species, like Schistosoma mansoni and Trichobilharzia ocellata, also follow a chemical trail laid by the secondary host. Because cercariae are nonfeeding organisms, they must find a host quickly. The strong-swimming species generally need to locate a host more rapidly than slow-crawling species, with swimmers surviving only about 24 hours without a host, while crawlers can continue living without a host for several days.

Among digenetic trematodes, the species of several families of blood flukes, including the Schistosomatidae, Spirorchiidae, and Sanguinicolidae, skip the second intermediate host; their cercariae invade the definitive hosts directly. A few, like those of the families Azygiidae and Faciolidae, encyst on vegetation rather than in a secondary host. The definitive host then becomes infected by eating the trematode-infested vegetation. Other trematodes, such as Alaria species, add a larval stage that m i

Light microscope image of a lancet liver fluke (Dicrocoelium dendriticum), a parasite of sheep. The eggs are picked up and carried by snails and the larva are transferred to ants through the snail's excretions. The maturing larvae cause cramping in the ant's mandible musculature which causes the ant to clamp down on blades of grass and get stuck. The larva are then ingested by herbivores such as sheep and cattle where they develop to the adult stage in the bile duct of the liver. Eggs are excreted by the sheep, starting the life cycle over again. (Photo by Oliver Meckes/Photo Researchers, Inc. Reproduced by permission.)

forms after the cercaria and develops into the metacercaria. This stage is called the mesocercaria. Plagioporus sinitsini is an atypical trematode that can apparently take any of three paths from egg to adult. In the most intriguing of the three, these trematodes are able to bypass their usual second intermediate and definitive hosts, and develop into adults while still in the sporocysts of the first intermediate host. As a result, they can produce and hatch eggs, and release already-developed miracidia through the feces of their first intermediate hosts.

Cercariae enter the secondary host, drop their tails, and develop into metacercariae. Metacercaria look like the adults, except they lack reproductive organs. Often, the host can survive, ostensibly ignoring even heavy infestations with metac-ercariae. Young bluegills (Lepomis macrochirus ), on the other hand, die if they are infected with too many metacercaria of the species Uvulifer ambloplitis. In some cases, metacercariae can cause behavioral or morphological changes in the second intermediate host species that have the effect of making it more susceptible to predation by the definitive host. Trematodes in the genus Ribeiroia, for example, can trigger a frog to produce additional or fewer hind limbs. Such deformities effectively cripple the host, making it an easier target for birds, the definitive host.

Transmission to the definitive host usually results from the host's inadvertent ingestion of the metacercariae. From the digestive system, the metacercariae migrate to the target site, which varies by species, and reach their sexual maturity in that location.

Studies of trematode communities in the first intermediate host indicate that a single host carries no more than four different species of trematodes. The reason for this low number is under some debate. One hypothesis suggests that interspecific predation by redia keeps the number in check. Other researchers believe that temporal and/or spatial separation of trematode species accounts for the limit of four trematode species per host. There may also be limitations on space or other resources.

Members of the subclass Aspidogastrea have a simpler lifestyle than the digenetic trematodes. It usually involves just one host, often freshwater mussels or snails, in which development from egg to adult occurs. The larvae (cotylocidia) generally lack cilia, but some, like the larvae of Lophotaspis vallei, have cilia and are good swimmers. Nonciliated larvae, in contrast, creep rather than swim.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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