STUDIES ON BIOLOGY OF FRESHWATER MUSSELS. 233 



It was shown by Posner ('75), Wallengren ('05), and other?, 

 against the contention of M'Alpine ('88), that the collections of 

 food are transmitted to the labial palps, and by their cilia to the 

 mouth. The writer shows (I.e.] that the ciliary streams of the 

 upper portion of the mantle chamber all tend toward the mouth; 

 while those of the lower portion of the visceral mass and mantle 

 lead away from the mouth. The latter accomplish the duty 

 ascribed by M'Alpine to all the cilia, that of freeing the mantle 

 chamber of heavier materials and rejected food clots. 



It was stated by the writer that the food material is subject to 

 rejection at four points: (i) the siphons, (2) the point on gills 

 and mantle where the food stream passes to the palps, (3) the 

 furrowed surfaces of the palps, and (4) the lips, at the mouth. 

 More recent observations have all corroborated this. Perhaps 

 the fact has not been sufficiently stressed that only an unusual 

 chemical or tactual stimulus results in the closure of the siphons 

 or lips. The palps somewhat oftener refuse masses from the 

 gills and mantle by turning aside. The greater number of reac- 

 tions occur as the food stream passes between the contiguous 

 palp surfaces. 



The work of Wallengren ('05), and others, has demonstrated 

 the action of the labial palps, the ridges of which are capable of 

 reversing the food stream. Near the distal margin of each ridge 

 of the palp surface the beat of the cilia is toward the apex, both 

 in front and behind. When the ridges are inclined forward, the 

 effective beat of the cilia is forward ; when the ridges alter their 

 axis, the backward-beating cilia are brought into play and the 

 others turned under. 



The course of the ciliary streams at the bottoms of the furrows 

 between the ridges is much more difficult to observe. Wallengren 

 believes that the cilia strike downwards to the edge of the palps, 

 and that the resulting streams belong to the excurrent system. 

 Siebert ('13), working on Anodonta ccllensis, says they strike 

 in the opposite direction upward to the apex of the inverted V 

 formed by the two palps, thence forward to the lips and mouth. 



