The Rugogastridae are a taxonomic family of flatworm

Flatworm

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...

s that consist of a single genus, Rugogaster, with only two species. The rugogastridae are parasites, and infect the rectal glands of various species of holocephalan fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

. The family, genus and two species are characterised by a single row of rugae (transverse thickenings of the body surface), numerous testes, and two caeca. Species of the other three families of Aspidogastrea

Aspidogastrea

The Aspidogastrea is a small group of flukes comprising about 80 species. It is a subclass of the trematoda, and sister group to the Digenea. Species range in length from approximately one millimeter to several centimeters...

have a single caecum and either one or two testes.