I3O WILLIAM RAY ALLEN. 



substances. It is flanked above and below by the thin narrow 

 lips. The upper lip is continuous with the outer labial palp on 

 each side, while the lower lip is prolonged into the inner right 

 and left palps. Most of the ciliary currents of the contiguous 

 faces of the palps and of the lips are directed forward to the 

 mouth (Figs. 3 and 5). The outer or non-contiguous faces of 

 both palps and lips (Fig. 4) as well as the edge of the inner face 

 of the lips, bear cilia which are directed backward and away from 

 the mouth. Thus particles which find their way between the 

 palps are carried to the mouth. As will soon be seen, very little 

 undesirable matter ever reaches the mouth or palps, but even 

 here Wallengren ('05) has pointed out how selection and rejection 

 may be made. 



As shown diagrammatically in Fig. 4, the inner surface of the 

 labial palps, except their outer margins, are made up of minute 

 vertical ridges, or furrows. These constitute a quite complex 

 mechanism for the sorting of material. In cross section (length- 

 wise of the labial palps) they appear as in Fig. 5. 



Upon the ridges as elsewhere occurs a ciliated epithelium. But 

 the ciliary currents are disposed in a unique manner. Upon the 

 anterior slope of each ridge they are directed backward (Fig. 5, p) 

 while those on the posterior slope lead forward (Fig. 5, a). This 

 seeming conflict is not such in fact, because only one set of cilia 

 comes into action at a time. The position of the ridges deter- 

 mines which set shall function at a given moment. Their normal 

 position seems to be that seen in the two ridges on the right in 

 Fig. 5, a somewhat reclining one, overlapping one another toward 

 the anterior. Thus the after slopes (a) are ordinarily brought 

 uppermost, the ciliary currents leading to the mouth are upon 

 the surface, while the cilia (p) which lead from the mouth lie 

 somehwat underneath the ridges. So long as no adverse stimuli 

 are received, particles which lie between the palps are thought 

 to be passed on forward from one ridge to another, to the lips 

 and mouth. 



In the event that distasteful matter reaches the palps a reflex 

 erection of the ridges (Fig. 5, /) brings uppermost the cilia leading 

 backward (p) and such material is returned from summit to 

 summit to the edge of the palps and discharged into the mantle 

 chamber. 



