Lanista was the trainer of gladiators. The lanista’s art was probably to train people to fight, and then put them against each other in interesting combinations: for example, the combination of “fishhead” (mirmillo) with “net-man” (retiarius).
From time to time, the lanista hired gladiators from other teams to make up for the losses or get the famous player for a while. Lanista probably also trained gladiators to fight wild animals and acquired animals for fighting.
