A cassette for collecting tissue samples from a fluid stream, said cassette comprising: a body (64, 72, 78), the body having: a fitting for receiving a suction line through which fluids are drawn away from a patient; an outlet opening through which a suction is drawn; a tray-receiving void between the fitting and the outlet opening, the 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 forming a bypass fluid path around the 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; and a valve (124, 188) movably mounted to the body to receive the fluid drawn into the fitting, the valve adapted to directed the fluid flow from the fitting into either the bypass conduit (88, 89) or the tray-receiving void, characterized in that: the cassette body (64, 72, 78) is formed with an exposed face and comprises plural tray-receiving voids that extend inwardly from the exposed face; the cassette comprises separate catch trays each removably mounted in one of the tray-receiving voids such that each catch tray is independently removable from and reinstallable in the tray-receiving void in which the catch tray is held; and the valve (124, 188) is configured to direct the fluid flow from the fitting into either the bypass conduit (88, 89) or any one of the tray-receiving voids receiving a respective one of the separate catch trays. a body (64, 72, 78), the body having: a fitting for receiving a suction line through which fluids are drawn away from a patient; an outlet opening through which a suction is drawn; a tray-receiving void between the fitting and the outlet opening, the 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 forming a bypass fluid path around the 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; and a valve (124, 188) movably mounted to the body to receive the fluid drawn into the fitting, the valve adapted to directed the fluid flow from the fitting into either the bypass conduit (88, 89) or the tray-receiving void, characterized in that: the cassette body (64, 72, 78) is formed with an exposed face and comprises plural tray-receiving voids that extend inwardly from the exposed face; the cassette comprises separate catch trays each removably mounted in one of the tray-receiving voids such that each catch tray is independently removable from and reinstallable in the tray-receiving void in which the catch tray is held; and the valve (124, 188) is configured to direct the fluid flow from the fitting into either the bypass conduit (88, 89) or any one of the tray-receiving voids receiving a respective one of the separate catch trays. the cassette body (64, 72, 78) is formed with an exposed face and comprises plural tray-receiving voids that extend inwardly from the exposed face; the cassette comprises separate catch trays each removably mounted in one of the tray-receiving voids such that each catch tray is independently removable from and reinstallable in the tray-receiving void in which the catch tray is held; and the valve (124, 188) is configured to direct the fluid flow from the fitting into either the bypass conduit (88, 89) or any one of the tray-receiving voids receiving a respective one of the separate catch trays.