XL] THE FRESH-WATER MUSSEL. 109 



and end, ventrally, in the thickened margins already men- 

 tioned. They pass into one another in front of the mouth ; 

 at the sides, they are united with the dorsal edges of the 

 outer gill-plates; and, behind, they extend upwards and on 

 to the dorsal face of the body, before finally passing into one 

 another above, and in front of, the anus, which is small, 

 tubular, prominent, and median. Thus the anus is inclosed 

 in a part of the cavity bounded by the two mantle lobes, 

 which is relatively small and shallow, and is termed the cloacal 

 chamber; while the gills, the foot, and the palps, hang down 

 into the relatively large branchial chamber which occupies 

 the space between the mantle-lobes for the rest of their 

 extent. It is the prolongation of the margins of the former 

 cavity which gives rise to the tubular anal siphon seen in so 

 many Lamellibranchs ; while the ventral or branchial siphon 

 is a similar prolongation of the margins of the branchial 

 chamber. The dorsal siphon is the channel through which 

 the exhalent currents pass ; the ventral, that for the inhalent 

 currents. 



The currents are produced and kept up by the action of 

 the cilia which abound upon the gills. The latter are per- 

 forated by innumerable small apertures, and the chambers 

 contained between the two lamellae of which each gill is 

 formed, are in communication, above, with the cloacal 

 chamber. The cilia work in such a way as to drive the 

 water in which the animal lives from the outer surface of 

 each gill towards its interior. Hence the current which sets 

 from the branchial to the cloacal chamber. 



The current of water which is thus continually drawn into 

 the branchial chamber carries with it minute organisms, In- 

 fusoria^ Diatoms and the like, and many of these are swept 

 to the fore part of the branchial chamber, where they enter 

 the mouth, and are propelled by the cilia which line its cavity 



