Heterotardigrada

Heterotardigrada
Echiniscus sp.
Scientific classification
Kingdom:	Animalia
Phylum:	Tardigrada
Class:	Heterotardigrada Marcus, 1927
Families
Order Arthrotardigrada   Archechiniscidae   Batillipedidae   Coronarctidae   Halechiniscidae   Renaudarctidae   Stygarctidae Order Echiniscoidea   Echiniscidae   Echiniscoididae   Oreellidae

The class Heterotardigrada includes tardigrades (water bears) that have cephalic appendages and legs with four separate but similar digits or claws on each. 444 species have been described.^[1]

The anatomy of the reproductive system is an important defining feature in distinguishing the different groups of tardigrades. Heterotardigrades have gonoducts that open to the outside through a preanal gonopore, rather than opening into the rectum as in the only other confirmed class of tardigrades, the Eutardigrada (the third class, Mesotardigrada, is represented by a single species whose reference material was destroyed in an earthquake, so its reproductive anatomy has not been studied recently).

Some orders of heterotardigrades are marine, others are terrestrial, but as for all tardigrades, all are aquatic in the sense that they must be surrounded by at least a film of moisture in order to be active - though they can survive in a dormant state if the habitat dries out.

References

• "Heterotardigrada". Integrated Taxonomic Information System.
• "Arthrotardigrada". Integrated Taxonomic Information System.
• "Echiniscoidea". Integrated Taxonomic Information System.

1. Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12.