Miracidium

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As Miracidium or eyelashes larva is called a life stage of parasitic trematodes (Trematoda) and monogenea (monogenean), in the latter case it is, however, mostly as Oncomiracidium referred. Examples of species are the liver fluke and the pair leech .

These are free-swimming, ciliated larvae that are used to spread in the water and to infect a suitable host . In the case of suction worms, this is the (first) intermediate host, usually a species of snail , in the case of hooked suction worms, the final host, usually a fish , more rarely an amphibian or reptile .

Miracidia hatch from eggs, which are usually released into the water by the final host with the faeces. At the time of release, the eggs are already fertilized and often have a lid ( operculum ) that can be blown off when hatching.

Anatomically, miracidia are characterized by excretory organs called protonephridia and a lack of digestive system; they obtain their metabolic energy from egg yolks. But they have a nervous system with a cerebral ganglion and various sensory organs that serve to track down the intermediate host. The rear half of the larva contains numerous stem cells that have emerged from an original stem cell formed after the first cell division of the fertilized egg and are used exclusively for reproduction. Even in the Miracidium, through continued cell division of these unfertilized stem cells, embryos are formed in an asexual way, which are composed of somatic cells (normal body cells) and other stem cells.

As soon as miracidia encounter a suitable host, it is infected, for example through the intestine, with the help of special penetration structures. The ciliated epidermis is now replaced by a special outer skin, the neodermis , which is characteristic of parasitic flatworms . A metamorphosis to the next stage of the life cycle, the sporocyst , follows.

Hooked suckers show no host change; here the larvae, usually called oncomiracidia, immediately attack the ultimate host. They characteristically have a special structure at the rear end, the haptor , which is provided with numerous hooks and suction cups, all of which serve to anchor in the host.