Note: Do not rely on this information. It is very old.
Abdominalia, a sub-order of Cirripedia, the members of which live as parasites in Mollusca or other Cirripedia. Parasitism has as usual produced degeneration, which is especially marked in the males. Alcippe, one of the best known genera, is common on the English coast, frequenting the shells of whelks and similar molluscs.