27. 



The currents of water are probably for respirator7 pur- 

 poses, 830 d to keep the 'nantle-chanber clear of the dirt that is 

 constantly finding its way in, r^ore especially the faeces, Aifiiich, 

 being so largely composed of sand and other hea^^?- particles, 

 would otherxvise drop into, and soon clog, this chamber. 



It seems to be a ju.stifiable siiggestion that the con- 

 tract i02i of the gills may aid in the movements of blood as well 

 as/. water. 



The blood spaces of the plates, and especially of the 

 suspensory raembran.es are considerably diminished in size during 

 each contraction 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 modi- 

 fy 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 ms-ntle chamber, in which 

 they form a moveable partition. Contact is insured by the pres- 

 sure of the blood inside the plates, an.d by the soft dorsal pro- 

 jections. 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. 



Ao the .Q J- llg lio at ro -s t- b efore th r e - •«©*»*^:'«;e%4.-o*i 



