of segmented animals, especially arthropods. In the ancestral arthropod, the body was made up of repeated segments, each with similar internal organs and appendages. One evolutionary trend is the grouping together of some segments into larger units, the tagmata. The evolutionary process of grouping is called tagmosis (or tagmatization).

The first and simplest stage was a division into two tagmata: an anterior "head" (cephalon) and a posterior "trunk". The head contained the brain and carried sensory