xi.] THE FRESH-WATER MUSSEL. 115 



ventral siphon open below and continuous with 

 the cleft between the ventral edges of the 

 mantle-lobes. 



e. The branchial or pallial chamber: turn back 

 the ventral edge of that mantle-lobe from which 

 the shell has been removed : a chamber is thus 

 exposed into which the ventral siphon, and the 

 cleft continuous with it, lead. 



. The cloacal chamber: pass a probe through the 

 dorsal siphon; it will enter a small chamber, 

 separated from the pallial chamber by a par- 

 tition which unites the hinder part of the two 

 inner gills (c. fi). 



c. The contents of the pallial chamber. 



a. The foot: a large, yellowish, somewhat plough- 

 share-shaped mass, in the middle line; its apex 

 directed forwards and ventrally, towards the 

 front of the cleft between the mantle-lobes. 



/?. The gills or branchitz : two lamellar bodies 

 on each side of the foot, but reaching farther 

 back than it does : the outer gill on each side, 

 attached to the mantle-lobe; the inner, attached 

 to the foot in front, but farther back, separated 

 by a cleft from it; and behind the foot, united 

 across the middle line with its fellow so as to 

 form a partition separating the cloacal from 

 the pallial chamber. 



y. The labial palps : a pair of small triangular 

 processes on each side, in front of the gills 

 and on the dorsal end of the anterior edge of 

 the foot. 



82 



