680 JAMES i. KELLOGG 
represented in figures 51 and 52. In the former the general 
current on the folds is seen to be obliquely forward and markedly 
downward across them. Between the folds are dorsally directed 
currents which seem always to be exposed, not simply by the 
parting of folds in response to the stimulus of large quantities 
Fig. 55 Zirfaea gabbi; ciliation of palps and visceral mass; f, foot; bg, gill 
bases; m, mantle wall; p, posterior palp; vms, visceral mass. 
Fig. 56 Zirfaea gabbi; section across siphon; es, excurrent tube of siphon cut 
open dorsally; g, gills; 7s, incurrent tube of siphon. 
Fig. 57 Barnea costata; b, bay of mantle; cm, collecting membrane; es, ex- 
current tube of siphon; f, foot; ga, line of gill attachment; 7s, incurrent tube of 
siphon; m, mantle wall; vms, visceral mass; x, point at which material from 
visceral mass passes to collecting membrane. 
