Thompsoniidae is a family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 of parasitic

Parasitism

Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

 barnacle

Barnacle

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...

s. They belong to the bizarre and highly apomorphic

Synapomorphy

In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...

 superorder Rhizocephala

Rhizocephala

Rhizocephala are derived barnacles that parasitise decapod crustaceans. Their bauplan is uniquely reduced in an extreme adaptation to their parasitic lifestyle, and makes their relationship to other barnacles unrecognisable in the adult form...

, and therein to the more diverse of the two orders

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

, the Akentrogonida.

The Thompsoniidae are one of the smallish families of Rhizocephala, as typical for the Akentrogonida. They only contain four genera

Genera

Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...

, of which two are not universally accepted:

• Diplothylacus Høeg & Lützen, 1993 (disputed)
• Pottsia Høeg & Lützen, 1993 (disputed)
• Thompsonia
  Thompsonia (barnacle)
  Thompsonia is a genus of barnacle which has evolved into an endoparasite on other crustaceans, including crabs and mantis shrimp. It spreads through the host's body as a network of threads, and produces many egg capsules which emerge through joints in the host's shell...
  
  Häfele, 1911
• Thylacoplectus Coutière, 1902