G. A. DREW ON YOLDIA LIMATULA, SAY. 15 



as Kellogg 5 has pointed out, are so powerful in their action as fre- 

 quently to cause excised gills to move about in the water. As the 

 gills descend, the water passes between the plates. After reaching their 

 greatest ventral depression the gills may remain quiet for a longer 

 or shorter time. The period of rest is followed by energetic contractions 

 of the suspensory membranes, resulting in a vigorous discharge of water 

 through the exhalent siphon, as the chamber dorsal to the gills 

 diminishes, and a corresponding influx of water through the inhalent 

 siphon, as the chamber ventral to the gills enlarges. During this 

 exchange, the siphons are made quite rigid, probably by having blood 

 forced into them. The movements of the siphons accompanying the 

 movements of the gills are very conspicuous and have been mentioned 

 by Brooks 1 . 



The movements of the gills are more or less rhythmic, the time 

 varying with the needs of the animal. Thus the rhythm is much 

 faster after a disturbance that has caused all movements to be sus- 

 pended for some time, than when no such disturbance has occurred. 



The currents of water are probably for respiratory purposes, and 

 to keep the mantle-chamber clear of the dirt that is constantly find- 

 ing its way in, more especially the faeces, which, being so largely 

 composed of sand and other heavy particles, would otherwise drop into, 

 and soon clog, this chamber. 



It seems to be a justifiable suggestion that the contraction of the 

 gills may aid in the movements of blood as well as the movements 

 of water. The blood-spaces of the plates, and especially of the suspensory 

 membranes are considerably diminished in size during each con- 

 traction, and, as the openings where the spaces of the suspensory 

 membranes communicate with the auricles are guarded by ridges that 

 probably act as valves, each contraction must modify the blood-supply. 



A little study will show 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 insured 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 its descent. 



When the gills are depressed, the plates are loosely opposed and 

 allow water to pass freely between them. In this position the gills 



