A cassette for collecting tissue samples from a fluid stream, the cassette comprising: a body (64, 72, 78) comprising: a fitting for receiving a suction line through which fluids are drawn away from a patient; an outlet opening through which a suction is configured to be drawn; a tray-receiving void between the fitting and the outlet opening, the tray-receiving void being in a fluid path with the outlet opening; and a bypass conduit (88, 89) between the fitting and the outlet opening, the bypass conduit (88, 89) forming a bypass fluid path around the tray-receiving void; a catch tray removably mounted in the tray-receiving void, the catch tray adapted to allow fluid flow therethrough into the outlet opening while retaining material above a certain size that is entrained in the fluid flow through the catch tray; a valve moveably mounted to the body (64, 72, 78) to receive the fluid drawn into the fitting, the valve adapted to direct the fluid flow from the fitting into either the bypass conduit (88, 89) or the tray-receiving void; and a stop positionable to (i) allow a suction to be drawn on the tray-receiving void through the outlet opening with the valve positioned to direct the fluid flow into the tray-receiving void, and (ii) block suction draw on the tray-receiving void from the outlet opening with the valve positioned to direct the fluid flow into the bypass conduit (88, 89), characterized in that: a lock feature is connected to and configured to be moved with the valve to be positioned to (i) block removal of the catch tray from the tray-receiving void with the valve positioned to direct the fluid flow into the tray-receiving void, and (ii) be spaced from the tray-receiving void so as to allow removal of the catch tray from the tray-receiving void with the valve positioned to direct the fluid flow into the bypass conduit (88, 89). a body (64, 72, 78) comprising: a fitting for receiving a suction line through which fluids are drawn away from a patient; an outlet opening through which a suction is configured to be drawn; a tray-receiving void between the fitting and the outlet opening, the tray-receiving void being in a fluid path with the outlet opening; and a bypass conduit (88, 89) between the fitting and the outlet opening, the bypass conduit (88, 89) forming a bypass fluid path around the tray-receiving void; a fitting for receiving a suction line through which fluids are drawn away from a patient; an outlet opening through which a suction is configured to be drawn; a tray-receiving void between the fitting and the outlet opening, the tray-receiving void being in a fluid path with the outlet opening; and a bypass conduit (88, 89) between the fitting and the outlet opening, the bypass conduit (88, 89) forming a bypass fluid path around the tray-receiving void; a catch tray removably mounted in the tray-receiving void, the catch tray adapted to allow fluid flow therethrough into the outlet opening while retaining material above a certain size that is entrained in the fluid flow through the catch tray; a valve moveably mounted to the body (64, 72, 78) to receive the fluid drawn into the fitting, the valve adapted to direct the fluid flow from the fitting into either the bypass conduit (88, 89) or the tray-receiving void; and a stop positionable to (i) allow a suction to be drawn on the tray-receiving void through the outlet opening with the valve positioned to direct the fluid flow into the tray-receiving void, and (ii) block suction draw on the tray-receiving void from the outlet opening with the valve positioned to direct the fluid flow into the bypass conduit (88, 89), characterized in that: a lock feature is connected to and configured to be moved with the valve to be positioned to (i) block removal of the catch tray from the tray-receiving void with the valve positioned to direct the fluid flow into the tray-receiving void, and (ii) be spaced from the tray-receiving void so as to allow removal of the catch tray from the tray-receiving void with the valve positioned to direct the fluid flow into the bypass conduit (88, 89).