16 UNITED STATES NATIONAL MUSEUM BULLETIN 29 6 



larval forms, and perhaps would not have become permanently estab- 

 lished. Of the remaining two males, perhaps tiny worm borings or 

 the odd shape of the burrow could have been responsible for a water 

 passage. 



In three instances the external mantle flap hole was sealed by 

 balanoid barnacles or the cases of serpulid worms. In each circum- 

 stance there were no males associated with the females, although the 

 females were large and would definitely be expected to possess males. 



Water Transport by the External Mantle Flap 



Water is pumped through the hole formed by the penetrance of the 

 external mantle flap to the outside of the shell. It is pumped by the 

 movements of the mantle against the burrow wall. These movements 

 are produced by the stroking of the thorax in the feeding process, 

 corresponding to the beating of the cirri of the thoracicans. 



The water is pumped from the outside of the shell in toward the 

 female, and out the main aperture around the lips of the mantle sack. 

 When carmine particles are added to the water they can be seen to 

 move in jerky beats into this hole. When the hole is plugged with ex- 

 cessive carmine particles a cleaning action is employed by forcing 

 water back out through the hole, forcing particles about one-fourth 

 millimeter away from the shell. 



With nineteen measurements on three animals at 17°C, the pumping 

 beats occur 31.2 ±.2 times a minute, and pull carmine particles for 

 a distance of 0.25 mm. Between beats the particles are motionless. 

 The average opening to the external mantle flap was 0.44 X 0.10 mm, 

 giving a minimum volume pumped of 0.011 mm^ per stroke. This 

 is a minimum figure, because the carmine particles are pulled from all 

 directions from 0.25 mm, and travel a greater distance within the 

 hole itself, although this distance would be impossible to determine 

 precisely. With 31.2 strokes per minute, there would be 0.343 mm ^ 

 of water per minute passing through the hole. 



If there were six males (a high number) with the female, each of 

 0.008 mm^ volume, in each minute about 7.2 times as much water 

 goes by as their combined volume. The average number of males per 

 female with males, of 305 barnacles, is 1.96 males per accompanied 

 female. This volume of water should be quite sufficient to supply the 

 males with ample fresh sea water. Of course, the female probably 

 uses some of the oxygen for her external mantle flap and the left side 

 of the mantle. 



