Paragonimus westermani has four major forms that should be considered. The first is the miricidia (free-living larva), the second is the cercaria (or larval worm), the third is the metacercaria (encysted worm), and the final is the adult fluke. Each form has unique characteristics and fascinating adaptations that allow it to thrive in each of the species’ several hosts.
The miricidia is the smallest form. It hatches from the egg in an auquatic environment, and uses its cilia to swim to its first host, the freshwater snail (Liu et al. 2008). There is not much information about the specific physical features of P. westermani miricadia, but the larger group digenea miricidia have an anterior opening leading to penetration ducts within, and a posterior region that contains structures that will aid in asexual reproduction later in life (Animal Diversity Web 2012).
Once inside their first intermediate host (Paludomus snails), the miricidia grow into cercaria and are 174.5 um in length and 73.2 um in width on average. The cercaria is oval shaped, has a sucker on one end, and a flagellate tail for locomotion on the other (Iwagami et al. 2007).
The metacercaria can live in several second intermediate hosts (freshwater crabs, crayfish), are oval shaped, can be thick or thin walled, and are larger than the cercariae (between 724 and 916 um in length, and between 598 and 771 um in width). The metacercariae can infect the muscles, hepatopancreas, and gills of the crabs (Rekah Devi et al. 2012). The metacercariae do not move, and once in the crab, they encyst (form a tough layer) around themselves. They each contain a single worm that will become the adult fluke in the definitive host (mammals). The metacercariae find their way to human (and othermammal) hosts when the mammal ingests the raw meat of infected crabs, boars, or sometimes (rarely) crayfish (Kuk-Na, et al. 2005).
Once inside the definitive mammal host, the metacercaria excysts (comes out of its cyst). The newly emerged metacercaria then does something very interesting. It begins secreting an enzyme that allows it to: evade the host immune system, intake nutrients from the host body, and dissolve the host tissue to aid in the movement of the metacercaria (Kuk-Na, et al. 2005). The metacercaria then begins traveling directly through the host intestines until it reaches the lungs. Once in the lungs, it forms a thick fibrous layer around itself, and begins developing into an adult fluke. (Rekha Devi et al. 2012).
The adult fluke is mostly flat and ovoid in shape, between ~6 -7mm in length, and between ~4-5 mm in width. The fluke has two suckers, one oral and 
one ventral (belly). The skin is covered in spines, and eggs laid by the fluke were dark brown in color (Rekha Devi et al. 2009). Perhaps the most interesting adaptation displayed by the adult fluke is its ability to thrive in both low oxygen and high oxygen environments. This adaptaion is necessary because (as previously stated) the fluke can end up not only in the lungs, but in many low-oxygen environments in the host body. The fluke can do this because it actually has three distinct populations of mitochondria! Each population serves to produce ATP most efficiently in different environments. (Takamiya et al, 2010). For another organism that employs INCREDIBLE adaptations to survive hostile environments, see this page.
one ventral (belly). The skin is covered in spines, and eggs laid by the fluke were dark brown in color (Rekha Devi et al. 2009). Perhaps the most interesting adaptation displayed by the adult fluke is its ability to thrive in both low oxygen and high oxygen environments. This adaptaion is necessary because (as previously stated) the fluke can end up not only in the lungs, but in many low-oxygen environments in the host body. The fluke can do this because it actually has three distinct populations of mitochondria! Each population serves to produce ATP most efficiently in different environments. (Takamiya et al, 2010). For another organism that employs INCREDIBLE adaptations to survive hostile environments, see this page. 
metacercaria of Fasciola hepatica which is the infective stage for the definitive host, usually sheep but also other ruminants and also occasionally man. 400x
typical gymnocephalus cercaria of a fascioliid. This is a ventral view showing the spherical acetabulum framed by the two branches of the caeca. This Fasciolopsis buski cercaria is indistinguishable from the cercaria of Fasciola hepatica. 100x
Ciclo de Vida de Schistosoma mansoni - La esquistosomosis Cercaria
Cercaria son la forma larvaria del parásito liberado del caracol huésped intermediario para todos los parásitos trematodos.
La cercaria de esquistosomas se caracterizan por tener largas colas que se bifurcados al final (los ramos Furcal). El tegumento se cubre con una membrana plasmática trilaminado, y en la superficie exterior con un glycocalyx, y está equipado con espinas y papilas sensorial.
Diagrama esquemático de cercaria schistosome
Para S. mansoni las cercarias se liberan 25 a 30 días después de que el caracol ha sido infectado. Se diferencian sexualmente, con macho y hembra cercarias. Se trata de una fase no alimentación del ciclo de vida, y sus requerimientos energéticos se cumplen por tanto glucógeno almacenado en la cola y el cuerpo.Como son no-alimentación, sus reservas de energía se agotará, resultando en una reducción de la infectividad, por lo general debido al agotamiento de las reservas de la cola ya que estos se utilizan primero, donde el cercaria se vuelve incapaz de nadar hacia su anfitrión. Cercaria puede permanecer infecciosa, en condiciones óptimas, para entre 5 y 8 horas después de derramar, aunque en el campo de esto es probablemente mucho menos, debido a factores tales como la variación en la temperatura del agua. Como con el miracidio, el cercaria presenta una serie de características de comportamiento que le permiten localizar su hospedador definitivo. Estas características incluyen las cercarias muestran estallidos de arriba natación para traerlos a la superficie del agua, seguido de períodos de hundimiento pasivo. Los cercaria también se ven afectados por otros estímulos tales como sombras en el agua, la turbulencia y productos químicos secretados por la piel hosts. Una vez que ha llegado a su huésped que concede a, a continuación, penetra en la piel humana, (utilizando las secreciones de las glándulas en la región de la cabeza) donde se despoja de su cola para convertirse en un schistosomulum larvas. Esto comienza una migración a través del cuerpo, hasta que se alcanza la madurez como un gusano adulto en el hígado. Cercaria de especies de esquistosomas que no completan su ciclo de vida en el hombre puede a menudo todavía penetrar la piel humana. Aquí pueden someterse a una migración a corto antes de morir, dando lugar a un tipo alérgica patología .
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