Cycliophora

The Cycliophora are a platyzoan phylum, based on a single genus Symbion. They are so different from other animals that they were put in their own phylum. They were discovered in 1995, and are the most recent new phylum. So far, three species have been found.
They live on the bodies of cold-water lobsters. They are microscopic: the feeding stage is about 0.3 mm long, and 0.1 mm wide.
Their life style is commensal, (a form of symbiosis) – they feed on the leftovers from the lobster's own meals.
Life stages.
They have three different body forms, and a two-stage life-cycle.
Reproduction.
"Symbion" can reproduce both asexually by budding and sexually. In sexual reproduction the male attaches to a feeding stage and impregnates a budding female. The female then separates from the feeding stage and attaches herself to another host, where the larva in her develops. The female dies, and the larva escapes.
The sexual reproductive cycle is triggered when the host crustacean moults its 'shell' (exoskeleton) in order to grow.