ANODONTA 61 



posteriorly into the cloacal chamber. A probe, passed back- 

 wards into the slit between the foot and the gills, will traverse 

 the supra-branchial passage of one side and emerge in the 

 cloacal chamber just beneath the posterior adductor muscle. 

 The cilia which cover the gill-bars cause a constant stream of 

 water to flow through the inhalant aperture into the branchial 

 chamber. The water passes either through the pores in the 

 gill-plates or by way of the slits between the foot and the inner 

 lamella of the inner gill-plates into the supra-branchial passages 

 and thence into the cloacal chamber and out by the exhalant 

 aperture. Any infusoria, diatoms, and other minute organ- 

 isms contained in the water are swept by the cilia of the gills 

 towards the labial palps. These, being richly ciliated, sweep 

 the organisms onwards down the grooves between the upper 

 and lower palps, and so into the mouth. The structure and 

 disposition of the gills of Anodonta, as described above, are very 

 complicated and difficult to understand. But in many bivalve 

 molluscs (the common sea mussel, Mytilus edulis^ is a good 

 example) the structure is less complicated, and by a comparison 

 of these and many other forms we are enabled to understand 

 how the complexity in Anodonta has come about. The primi- 

 tive gill of a mollusc has been compared to a comb with a 

 double series of teeth, and hence has been called a ctenidium. 

 It may be more aptly compared to a feather having a central 

 axis and a number of filaments arranged along two opposite 

 sides of the axis like the barbs of a feather. The axis is 

 attached to the body-wall, and its other end bearing the plume 

 projects freely backwards into the mantle-cavity. This con- 

 dition is realised in a large number of living molluscs. If now, 

 as frequently happens, the axis lies close to the body-wall, and 

 is fused to it for the greater part of its length, the two rows of 

 filaments will droop vertically downwards in the mantle-cavity, 

 and will represent the outer and inner gill-plates. This con- 

 dition again is represented in living molluscs. Further, if the 

 filaments become very long, are bent upwards again at an acute 

 angle, anci are connected together, they will form two flat gill- 

 plates, each comprising an outer and an inner lamella. In the sea 

 mussel, Mytilus edulis, the gills are in this condition, the fila- 

 ments being loosely locked together by patches of stiff cilia so 

 that they can be divided with great ease from one another. 

 The descending and ascending limbs of the filaments repre- 



