Distome (Leucochloridium paradoxum)
Sometimes amber snails (Succinea or Oxyloma) can be found, that attract attention by their largely swollen tentacles, laterally striped by pulsating rings. This phenomenon is because of an infection of the snail by a parasitic distome, Leucochloridium paradoxum.
This parasitic worm belongs to the group of flatworms (Plathelminthes), and among these to the distomes (Trematoda, Digenea).
Similar to other flatworms, the development of a distome also passes a change of generations, where one generation infects a snail as intermediate host, and one generation infects a warm-blooded animal, in this case a bird, as final host.
The bird, with its droppings, spreads the distome's eggs, of which hatch the ciliated larvae or miracidia. Those then infect a snail living near the water.
Out of the snail's digestive system, the miracidia wander into the main digestive gland and change into the next larval stage, the cercariae. Those gather in sporocystes, long tubes, that extend into the snail's tentacles. Several hundred cercaries can be contained in one of those sporocyste tubes.
The sporocyste tube is what can be seen pulsating in the snail's tentacles. The tentacles, swollen by the sporocystes, cannot be withdrawn by the snail, but instead attract a bird by their eye-catching colouring and movement. The bird rips off the snail's tentacle, and as a reward, is infected by the cercariae within.
In the bird's digestive system adult distomes evolve from the cercariae, that reproduce sexually and lay eggs, which are then spread by the bird's droppings. The circle is closed.
Oh yeah: and Bedbugs.