502 



teriorly where the foot and mantle join, and posteriorly where the gills 

 are united to the wall that separates the two siphons. 



In young specimens, the movements of the brown gills are visible 

 through the shell. The gills are gradually pressed ventrally, probably 

 by the blood forced into them, possibly aided by the powerful cilia. 

 As the gills descend the water passes between the plates. After a 

 period of rest, varying according to circumstances from a second to 

 a minute or more, the suspensory membranes contract vigorously, and 

 the gills quickly force the water that lies dorsal to them, out of the 

 exhalent siphon. The current of water thus created is strong enough 

 to carry small grains of sand, and other hard particles, to a distance 

 of some decimeters. In this way the fasces, which are composed so 

 largely of heavy particles, are swept from the mantle-chamber. 



It is quite possible that the movements of the gills may also aid 

 in the movements of the blood. The blood-spaces of the plates, and 

 especially of the suspensory membranes, are considerably diminished 

 in size during each contraction, and, as the spaces of the suspensory 

 membranes communicate with the auricles, each contraction must pass 

 some blood on into the heart. 



A little study shows how admirably the gills are fitted for the 

 function of pumping water. In shape, they exactly fit the mantle- 

 chamber, in which they form a movable partition. Contact is in- 

 sured by the pressure of the blood inside the plates, and by the 

 soft dorsal projections of the plates. These projections must act much 

 like the leather on the plunger of a suction pump, making good 

 contact when there is pressure from above, but not hindering descent. 

 As the gills lie at rest before the contraction of the suspensory mem- 

 branes, the plates are loosely opposed and allow water to pass freely 

 between them. In this position, the gills are much longer than they 

 are during contraction , and lie in curves , Fig. 1 g. When the 

 suspensory membranes contract, the longitudinal muscles also contract, 

 the gills are shortened, and the plates are more closely opposed. 



Each plate is rendered rather rigid ventrally by chitinous rods 

 that are disposed so as to take the strain in pumping water. Muscle- 

 fibers are continued from the suspensory membranes to these rods, 

 and keep the plates from bending ventrally when pressure is applied. 



The gills of Nucula are possibly used in creating slight currents 

 of water but the opaque shells of even young individuals render it 

 impossible to study the normal movements of the gills. It can be 

 determined, however, that slight currents of water are thrown out of 

 the shell and, as the gills are supplied with suspensory membranes 



