CILIARY MECHANISMS OF LAMELLIBRANCHS © 671 
are large, and move in response to the stimulus of much material, 
so as to expose downwardly directed tracts in the grooves be- 
tween them. 
Mantle. On the upper mantle wall, there is a trend downward 
and backward, and all collections are brought to and accumu- 
late at a point (6b) under the base of the incurrent siphon (s, 
figure 39). 
Pecten tirradians and P. tenuicostatus 
Pecten irradians is found from Cape Cod to Texas; Pecten 
tenuicostatus north of Cape Cod. ‘The ciliation of organs of the 
body is essentially similar in the two species, and they may be 
considered together; minor differences in the course of cilia 
streams will be pointed out. 
Gills. The organs are very extensive, and outer and inner 
demibranchs, on each side, are suspended by a thin, wide mem- 
brane. Figure 42 represents the outer demibranch of the left 
side of P. tenuicostatus, the gill membrane (gm) attached to the 
adductor muscle (a). The reference letter g is placed on the 
upper or basal edge of the outer lamella of the outer demibranch. 
The mechanism for disposing of collections on all lamellae is 
astonishing in its efficiency, and its adaptation seems to be 
perfectly clear. A description of it was given in my “Shellfish 
industries”? (’10) but perhaps should be repeated briefly here. 
The gill is plicated or folded; figure 48 gives a surface view of 
three of these plications, which are practically the same in all 
Pectens. Water, flowing between the filaments of the folds to 
the interior of the demibranch, brings suspended particles to the 
Fig. 42 Pecten tenuicostatus; gill ciliation; a, adductor muscle; g, gill; gm, 
suspending membrane of gill; x and y, points between which material is received 
from the under side of the adductor muscle. 
Fig. 43 Pecten irradians; gill mechanism for disposal of waste matter; a, 
carmine dropped on surface; b, same, rolled into a line; c, same, carried toward free 
margin; f, f, surfaces of folds; g, g, grooves between folds; m, m, tract leading 
toward palps. 
Fig. 44 Pecten irradians; palp and gill ciliation; ap, anterior palp; g, gill 
seen from below; mo, mouth; pp, posterior palp. 
Fig. 45 Pecten tenuicostatus; ciliation of visceral mass; a, adductor muscle; 
f, foot; n, nephridium; p, palp; vms, visceral mass; 7, rectum. 
