26. 



ent siphon. 



In young speciemns the niovenonts of the browi gills are 

 visible through the shell. They are gradually pressed ventrally, 

 probably by the blood forced into them, possibly aided by the 

 powerful cilia, which, as Kellogg has pointed out, are so 



powerfiil in their action, as frequently to cause excised gills to 

 move about in the water. As the gil is descend the water passes 

 between the plates. After reaching their greatest ventral de- 

 pression the gills may remain quiet for a longer or s}iorter tine. 

 Then comes a quick contraction of the suspensory mcanbranes, ac- 

 companied by 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 acco^npa-ying the move- 

 ments of the gills are very conspicuous and have been mentioned 



(1) 

 by Brooks ' . 



The movements of the gills are more or less rliythnic, 



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 suspended for sometime, than when, no such disturbance has 



occurred. 



