Microns 



250 



FiGfRE 106. — Longitudinal section of a portion of the 

 right external palp. The right (smooth) side is only 

 slightly ruffled. The left side is folded and forms deep 

 ridges and grooves. Bouin, hematoxylin-eosin. a.s. — 

 anterior slope of the ridge; g. — grooves; l.m. — longi- 

 tudinal muscles; p.s. — posterior .slope of the ridge; r. — 

 ridge covered with ciliated epithelium; t.m. — transverse 

 muscles. 



lobes. A nerve net was described in the palps 

 of Anodonta (Matthews, 1928). I was not able 

 to demonstrate its presence in the oyster palps. 



DIRECTION OF CILIARY CURRENTS 



The most conspicous features of the palps are 

 the abundance of mucous cells on their smooth 

 sides and the powerful ciliated layer covering the 

 ridges of the inner sides. The idea that labial 

 palps of bivalves represent a sorting apparatus 

 was clearly stated by Coupin (1893), who observed 

 that in Mytiluji, Cardium, and Pholas the more 

 voluminous particles are discarded by the palps 

 while the finest ones are delivered to the mouth. 

 He concluded that the principal function of the 



Microns 



200 



Figure 107. — Tangential section of the inner surface of 

 labial ])alp of C. virginica made through two and a half 

 ridges. The ruffled sides of the ridges are directed 

 anteriorly toward the mouth. Bouin, hematoxylin- 

 eosin. 



palps of Pholas is to prevent the bulky material 

 from reaching the digestive tract. 



The sorting of food is accomplished by a complex 

 system of ciliary ciu'rents along the surfaces of 

 the palps. With reference to C. virginica, Kellogg 

 (1915) states that the smooth sides of the two 

 internal palps facing each other direct the food 

 particles backward to the tips of the palps. This 

 statement is only partially correct. 



In describing the feeding habits of fresh-water 

 mussels Allen (1914) noticed that the ridges of 

 the inner surfaces of the palps normally overlap 

 one another in a reclining position. This is also 

 the usual position of the ridges in C. virginica 

 (figs. 106 and 108). On the anterior slope of each 

 ridge (fig. 106, a.s.) the currents are directed 

 toward the free end of the palp; the currents of 

 the posterior slope (p.s.) lead toward the mouth. 

 Thus the final direction of the movement of a 

 particle along the surface of the palp depends on 

 the position of the ridges. Allen thinks that, as 

 long as no adverse stinmli are received, the par- 

 ticles which lie between the palps pass on forward 



114 



FISH AND WILDLIFE SERVICE 



