Pyrex® brand filter flask, high standard series 5340.
This Pyrex brand 500ml filter flask features tubulation for easy connection to filter pump hoses. Heavy wall, graduated on side, flat bottom. Necks finished to give excellent fitting for #7 rubber stopper.
The filter flask looks like an Erlenmeyer flask with a short arm coming out of it. The "arm" is designed to connect the flask to a vacuum source. When sealed on the top with a stopper or a Buchner funnel, the vacuum flask will hold a reduced pressure. Note the heavy the walls of this flask - they are designed to withstand reduced pressure without imploding.
Filter flasks are also called "side-arm flasks" and "vacuum flasks" and are used either for filtering solutions or for removing solvent under reduced pressure.