198 THE FRESH-WATER MUSSEL [CH. VI 



individual only to seek another: occasionally it may be 

 seen running about upon the outside of the shell. A. 

 crassipes on the other hand quits its host after reaching 

 the second larval stage and lives free when adult. The 

 females of both species deposit their eggs in the gill tissue 

 of the mussel, and it is in the gills that the larval meta- 

 morphoses take place, but in the intervals between the 

 ecdyses the larvae quit the gill chambers and wander more 

 freely about the body. 



The eggs of other parasites have been recorded from 

 other parts of the body, viz., of Hydrocharis in Keber's 

 organ and of Aspidogaster in the pericardium. The adult 

 Aspidogaster is also found in the same cavity. The most 

 interesting of the parasitic organisms are however the 

 various species of Trematode Flatworms which pass their 

 immature stages within the body of the mussel, but only 

 become mature on reaching a second host, probably an 

 aquatic bird. The sporocysts and cercariae are found in 

 the spaces between the gut and the genital glands and 

 also in the mantle and pericardium, but the complete life- 

 history has not yet been worked out in any one instance. 

 The suggestion just put forward is based upon the analogy 

 of what is now known to be the case in the pearl-forming 

 sea-mussel, Mytilus edulis. 



Pearls. Pearls may be formed in almost any of the 

 tissues of the mussel, either singly or in groups ; they may 

 be loose in the tissues or may become attached to the 

 inner surface of the shell. In all cases they consist of 

 alternating concentric layers of calcareous matter and con- 

 chiolin, being in fact identical in structure with the inner- 



