CONTRACTIONS IN MANTLE — LAMELLIBRANCHS 233 



In Mya arenaria the shell may be held tightly closed without 

 disturbing the flow of water through the mantle chamber, for 

 the siphons can be extended when the valves are in this posi- 

 tion. The pulsations of the mantle in Mya arenaria are much 

 more powerful than those in Unio complanatus. A wave of 

 contraction can be seen to start in the distal end of the extended 

 siphon and move forward ending with the rise and fall of the 

 mantle. This recalls an observation of Dubois ('92) on Pholas 



Fig. 1 Kymographic records of the movements of the mantle and shell of a 

 Unio complanatus. The upper line (A) is a record of the movements of the 

 mantle; the second line (B) is a record of the movements of the shell. Both 

 records were made simultaneously. 



The third line (C) is a record of the movements of the mantle; the fourth 

 line (D) is a simultaneous record from the shell of the same specimen after the 

 shell had been rendered immobile by cutting away from the rest of the valve the 

 part of the shell which was attached to the adductor muscles. 



The lower line indicates time intervals of thirty seconds each. 



dactylus; a wave of muscular action in the walls of the siphons 

 served as a means of circulating water when the valves of the 

 animal were tightly closed. 



Figure 2, A, is a record of the mantle movements of Mya. 

 It shows that the interval between successive contractions is 

 about one minute, with often a longer pause after three con- 

 tractions. In table 1, is shown the rate of mantle and shell 

 contractions of Mya arenaria kept in running water during a 

 period of three days. This table indicates that the rate varies 

 considerably from time to time. 



Figure 3 is a record of the movements of the mantle lobes of 

 both sides of a Mya arenaria taken simultaneously. It will be 



