Abstract: A fluid treatment device has an exchangeable treatment unit and a connection head. A bayonet-type connection device connects connection head and treatment unit to each other by a plug-in/rotation movement about a connection axis of the treatment device and is provided with a housing connection part and a head connection part that operatively engage each other to form a connection of the bayonet-type connection device. Housing connection part and head connection part each have a mutually complementary functional element extending circumferentially about the connection axis and operatively connectable to each other. One functional element has an assembly ramp passing into a first support region perpendicular to the connection axis. The other functional element has a slide region adjoined by a second support region perpendicular to the connection axis. In connection end position, first and second support regions are positioned on top of each other relative to the connection axis.