Ml llimeters 



Figure 108. — Longitudinal section of the palps along the 

 lateral groove at the place of entrance to the mouth. 

 l.g. — lateral groove; l.m. — longitudinal muscles; m. — 

 mouth; r. — ridges of the inner sides of palps. Bouin, 

 hematoxyli n-eosin. 



from one ridge to another and eventually reach 

 the mouth. In case of hritation the reflex erec- 

 tion of the ridges brings uppermost the cilia 

 leading backward, and the particles on the surface 

 of the palp are pushed from summit to summit 

 to the edge of the palps and discarded. It is not 

 clear from Allen's description whether his expla- 

 nation is based on observations or on an assumption 

 that the ridges are capable of changing their 

 position in response to stimulation. 



During observations of the feeding of an oyster 

 spat less than 3 mm. long and attached to a glass 

 slide, Nelson (192.3) found that the filaments of 

 the palps narcotized in magnesium sulfate solu- 

 tion lose tlieir power of erection with the result 

 tliat large masses of material passing over the 

 filaments accumulated near the mouth and blocked 

 it. This effect may have been caused by the in- 

 hibition of ciliary motion as well as by the sup- 

 pression of muscular contraction. 



The "filaments" of the palps of the spat ob- 

 served by Nelson undoubtedly correspond to the 

 ridges of the inner surfaces of the palps of adult 

 oysters. The theory of sorting out of the material 

 advanced by Allen and accepted by Nelson implies 

 that the change in the direction of movement of 

 the particles toward the mouth or away from it is 

 controlled by the erection of the ridges. With a 

 microscope I examined the palps of oysters intact 



THE LABIAL PALPS 



except for the removal of one valve and was not 

 able to observe changes in the position of the 

 ridges. Similarly, no erection of ridges was 

 noticed on the excised palps which were kept in sea 

 water and examined with high power. The ciliary 

 Clu-rents remained, as a rule, undistm'bed for 

 many hours. 



Tonge (1926) states that in 0. ediilis "there are 

 no muscles within the folds such as could cause it 

 to contract downwards." This statement cannot 

 be confirmed by my observations on C. virginica. 

 In the palps of the American osyter the longi- 

 tudinal muscles are clearly visible (figs. 106 and 

 108, l.m.). They extend along both sides of a 

 ridge from its base to the very tip. The contrac- 

 tion of muscle fibers on one side of the ridge pre- 

 sumably may change the position of the ridge and 

 make it stand at right angles to the sm-face of the 

 palp. There is no evidence that this actually 

 happens. 



Churchill and Lewis (1924) arrived at the con- 

 clusion that in fresh-water mussels the cUia cover- 

 ing the upper portions of the ridge beat forward 

 (toward the mouth) while in the deeper part of 

 the groove between the ridges they beat in the 

 opposite direction. They also make a generaliza- 

 tion that "the palps appear to be a mechanism for 

 reducing the c}uantity of material to an amount 

 that can be handled by the mouth." According 

 to these authors the two sides of the palps perform 

 distinctly different functions. In fresh-water 

 mussels the ciliary currents along the outer 

 (smooth) surfaces remove the particles from the 

 mantle chamber, while on the ridged surfaces the 

 currents are directed toward the mouth. 



The currents along the labial palps of C. 

 rirginica are, however, more complex and some- 

 what different from those described for fresh-water 

 mussels. To observe these currents I removed 

 one valve, dissected the mantle hood, and pushed 

 apart its sides to expose the entire surface of the 

 palps. I placed the oyster in sea water in a 

 sliallow tray and observed it with a low-power 

 microscope under strong illumination directed at 

 an angle of about 20 degrees to the surface. 

 Suspensions of colloidal carbon or finely powdered 

 willemite in sea water were used to flood the sur- 

 face, and the distribution of the brilliantly fluores- 

 cent willemite particles was observed under 

 ultraviolet light. A series of pencil diagrams 

 outlining the positions of the palps were prepared 

 in advance and were used to mark the direction 



115 



