In the integrated car seat with a lengthwise adjustment mechanism having two pairs of rails, with a bottom support frame having two front pivotal supports, two rear pivotal supports and two side parts carried on the pivotal supports, with a seat back connected to the side parts by a seat back joint and with a seat belt buckle, the side part is configured in the shape of a shell on the seat side that is turned away from the buckle, the side part having a bottom and a perimeter profile rim. The side part turned away from the buckle is open toward the outside, the outside preferably meaning the outboard side of the seat.