499 



is supplied with a long appendage, paj), which springs from its posterior 

 end, near its dorsal margin. In young specimens these appendages 

 are flat and entirely confined to the outer palps. As the appendages 

 grow, each folds longitudinally so as to form a groove on its inner 

 side and, at the same time, twists so that it may appear like the 

 united dorsal margins of the outer and inner palps. Nucula has an 

 enlargement where the appendage joins the palp. The grooves are 

 lined with ciliated epithelium, and are continuous the whole length of 

 the appendages. 



Specimens of Yoldia, while feeding, Fig. 3, are generally slightly 

 tipped ventrally from the perpendicular, and frequently have about 

 one-third of their posterior 

 ends above the mud. While 

 in this position, the palp- 

 appendages, pap, are thrust 

 out of the shell and one, at 

 least, bends over and in- 

 serts its tip in the mud. 

 The cilia in the longitudinal 

 groove rapidly elevate the 

 mud, which is rich in living 

 organisms, along the groove, 

 and finally pass it between the 

 palps. From this position it 

 is passed to the mouth by 

 the cilia between the palps. 



Fig. 3. 

 halent siphon. 



An adult specimen of Yoldia limatula as it appears while feeding, es ex- 

 is inhalent siphon, pap palp-appendages, st siphonal tentacle. 



As both of the species of Nucula normally live entirely covered 

 by mud, it has not been possible to observe them collecting food under 

 normal conditions. If they are placed in dishes containing only mud 

 enough to partially cover them, the palp-appendages will be seen to 

 function as in Yoldia. 



Thus we find that Mitsukuei's (5) surmise, based on finding sand 

 in the grooves of the palp- appendages, was right. The palp-appendages 

 are food collectors. 



Gills. 



The following description will apply to the gills of Yoldia only. 



Each gill consists of a double series of rather broad flat plates, 

 placed laterally edge to edge, and antero-posteriorly with their flat 



