e impeller unit a lower clam shell 50 is joined to an upper clam shell 51 by a hinge 52 a disposable cassette 53 sets into lower clam shell 50 when the shell is in the open position fig1 and is clamped between lower and upper clam shells 50 and 51 when in the closed position fig1 cassette 53 and lower clam shell 50 may have the same overall shapes as in the previous embodiments upper clam shell 51 may be a continuous ring since it receives inlet port 54 axially the clam shell embodiment of fig1 and 16 does not accommodate a pump head cassette that is pre installed with closed loop tubing of a frame pack for example because of the need to insert inlet port 54 axially through upper clam shell 51 however a clam shell is useful in other applications of extracorporeal blood circuits such as a temporary cardiac assist application as shown in fig1 a venous cannula 55 for removing blood from a patient is coupled to inlet port 54 by a tube 56 an arterial cannula 57 is coupled by a tube 58 to pump outlet port 60 since the tubing does not form a closed loop cannula 55 and tube 56 can be u201c snaked u201d through the central hole in upper clam shell 51 and then cassette 53 can be closed between clam shells 50 and 51 advantageously a single latching locking mechanism not shown can securely hold this type of assembly in its closed position disposable plastic shields can be provided on the clam shells such as a.