Tantulocarida is a highly specialised group of parasitic crustaceans that consists of about 33 species, treated as a class in superclass Multicrustacea.

[4] The tantulus larvae has a head with a ventral oral disc but no appendages, a six-segmented thorax with six pairs of legs, and a limbless abdomen consisting of one to six segments in addition to a telson.

[5] Members of this subclass are minute – less than 0.3 millimetres (0.012 in) in length and have a dramatic reduction in body form compared to other crustaceans, with an unsegmented, sac-like thorax and a much reduced abdomen.

[6] One tantulocarid species, Tantulacus dieteri, is the world's smallest arthropod, with a total body length of only 85 micrometres (0.0033 in).

The tantulus larva transforms directly from a non-feeding (lecithotrophic) and free-swimming organism into a parasite without any instars.