The Temnocephalida are an order of turbellaria

Turbellaria

The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes , and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to in length...

n flatworms.

Unlike most other turbellarians, all the species in this order are either commensal

Commensalism

In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...

 or parasitic. They can be found living on crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, molluscs, and, in some species, even turtle

Turtle

Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s. The commensal species typically live in the gill or mantle cavities of their hosts, while the parasites live inside the digestive system. Anatomically, temnocephalidans can be distinguished from related groups by the presence of an adhesive disc on the underside for attachment to the host, and of a number of finger-like projections arising from the head.