V.] THE FRESH-WATER MUSSEL. 33 g 



y. The exoskeleton. A continuous investment for 

 the exposed surfaces of the mantle. It consists 

 of a pair of calcareous valves, which pass — 

 dorsally into a comified ligament — ventrally into 

 a chitinous free-border, which is inflected on to the 

 edges of the pallial muscles. 

 3. The branchice. Their descending lamina ; that of 

 the outer gill confluent with the pallium, that of 

 the inner gill in part confluent with the body 

 wall and with its fellow of the opposite side: 

 their reflected lamince ; confluent along their line 

 of mutual apposition. Note, in relation to the 

 latter and to the supra-branchial chamber, the 

 suspensory ligament of the gills. (Cf. Sect. B. c, 

 8, and d, /?.) 



e. The body-cavity. This is almost obliterated; 

 note the large pericardial chamber, situated in the 

 dorsal middle line. 



£. The viscera. The cut edges of the intestine will 

 be seen, lying within the body and traversing the 

 pericardial chamber. 



The excretory organ; median and paired, lying 

 immediately beneath the pericardial chamber. 

 The glandular segments of opposite sides are 

 seen to be approximated in the middle line; 

 note the reno-pericardial apertures lying imme- 

 diately above and internal to the attachments of 

 the inner gill lamellae. The muscular segments 

 are here confluent in the middle line, enclosing 

 a spacious cavity; dissect to the level of the 

 excretory orifice of one side, and note that it is a 

 perforation of the floor of this segment, opening 



