Millimeters 



Figure 113. — Tangential section across the mouth and 

 palps. Semidiagrammatic. e.g. — central gutter; l.g. — 

 lateral gutter; 1.1. — lower lip; l.i.p. — left inner palp; 

 1.0. p. — left outer palp; m. — mouth; mn. — mantle; r.i.p. — 

 right inner palp; r.o.p. — right outer palp; u.l. — upper 

 lip. Formalin and hematoxylin. 



the base and was carried toward the mouth. 

 Another particle (lower part of fig. 112) fell into 

 the furrow; several times it was kicked over two or 

 three ridges, but it always fell to the bottom of a 

 furrow and finally was carried toward the edge of 

 the palp and discarded (heavy arrow). Fre- 

 quently the medium-sized particles entangled in 

 mucus are caught in an eddy between the ridges 

 and begin to rotate in a turbulent current. Addi- 

 tional particles are entangled, and as the ball 

 grows it is kicked out and caught by the current 

 along the edge. There was no evidence of the 

 reversal of ciliary motion and the pattern of 

 currents described above was maintained through- 

 out the periods of observations. 



An effective sorting mechanism, which prevents 

 all but the smallest particles from reaching the 

 mouth, is provided by the system of ciliary move- 

 ment. The main supply of food is delivered by the 

 lateral gutters (fig. 113), which lead directly into 



the corners of the mouth. Only a small amount 

 of food reaches the mouth via the central gutter 

 where it must be pushed over the edge of the lower 

 lip. Observations of the ciliary motion of the 

 palps give one the impression that the number 

 of food particles delivered to the mouth constitutes 

 only a small fraction of the total quantity which 

 reaches the palps. It appears that the principal 

 function of the palps is to eliminate a large part of 

 food gathered in feeding and allow only a very 

 small portion of it to reach the digestive tract. 

 The system of currents and countercurrents ob- 

 served in the palps of C virginica is much more 

 complex than that described by Yonge (1926) for 

 the palps of O. edulis. 



Mucus plays an important role in the sorting of 

 food. The discharge of mucus is directly propor- 

 tional to the amount of particles which settle on 

 the surface of the palps. The sheet of mucus is 

 pulled over the ridges of the palps by the action 

 of the more powerful cilia at the tops of the ridges. 

 The smaller cilia of the grooves may not be in 

 direct contact with the mucus sheet above them 

 and beat in a different direction. Only the small- 

 est particles, free of mucus, are moved directly by 

 the cilia. 



A study of the ciliary currents of the palps 

 suggests that the food-sorting mechanism of the 

 oyster is designed for continuous feeding in water 

 containing a low concentration of minute food 

 organisms. Under such conditions only a limited 

 amount of mucus is used for sorting out the suit- 

 able forms. The question of whether the oyster 

 is capable of selecting its food on some other basis 

 than size has not yet been adequately studied. 



REACTION TO STIMULI 



The palps respond both to mechanical stimula- 

 tion and to the chemical stimulation of weak acids, 

 solutions of mineral salts, etc. If a weak mechani- 

 cal stimulus is applied to the edge of the palp, the 

 affected portion contracts, making a shallow in- 

 dentation which is proportional in size to the in- 

 tensity of the stimulus. ^Strong mechanical 

 stimulus or application of such irritating solu- 

 tions as weak hydrochloric acid, ethyl alcohol, 

 adrenaline, etc., provoke general contraction of 

 the palps and gradual curling up of their edges. 

 The external palps bend to the outside, while the 

 internal palps curl in the opposite direction and 

 seal the access to the central gutter (fig. 114). 

 Autonomous responses of the labial palps of Ano- 



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FISH AND WILDLIFE SERVICE 



