THE FRESH-WATER MUSSEL. 33$ 



The exoskeleton. Observe that this forms, during 

 life, a continuous investment for all the exposed 

 surfaces of the body, with the exception of the 

 foot. It consists of a pair of calcareous valves, 

 which pass dorsally into a cornified ligament 

 ventrally into a chitinous free-border, which is in- 

 flected on to the edges of the pallial muscles. 



The branchice; two pairs of lamellate organs, 

 confluent with each other and the body-wall 

 internally and with the pallial-lobes externally. 

 Examine their central cavities and attachments, 

 noting especially the subdivision of the supra- 

 branchial chamber into two, by the suspensory 

 ligament of the gills. (Cf. Sect. B. d, p.) 



The body-cavity. This is almost obliterated by 

 the great development of the reproductive gland; 

 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 



