Pygidium
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The pygidium is the posterior body part or shield of crustaceans and some arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor. It is composed of fused body segments, sometimes with a tail, and separated from thoracic segments by an articulation.
In trilobites, the pygidium can range from extremely small (much smaller than the head, or cephalon) to larger than the cephalon. They can be smooth, as in order Asaphida, or spiny, as in order Lichida.
Pygidium is also a superseded genus of fish of the family Trichomycteridae. Most species of this genus have been reassigned to the genus Trichomycterus.
References
- Some of the information in this article was taken from the website "A Guide to the Orders of Trilobites (http://www.aloha.net/~smgon/ordersoftrilobites.htm), by Sam Gon III.de:Telson