A cassette for collecting tissue samples from a fluid stream, the cassette comprising: a body 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 between the fitting and the outlet opening , the bypass conduit 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 to receive the fluid drawn into the fitting , the valve adapted to direct the fluid flow from the fitting into either the bypass conduit 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 , 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 . a body 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 between the fitting and the outlet opening , the bypass conduit 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 between the fitting and the outlet opening , the bypass conduit 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 to receive the fluid drawn into the fitting , the valve adapted to direct the fluid flow from the fitting into either the bypass conduit 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 , 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 . <EOS>