100 PEARLS [CH. VIII 



The adult worm (which he considered to be Leuc'i- 

 thodendrimn somateriae) lives in the Scoter (Oedemia 

 nigra) and there produces its eggs. The eggs pass 

 out of the bird to the external world, where probably 

 most of them die. The fortunate ones, however, eithei' 

 themselves become, or give rise to a tiny free-swimming 

 larva, which reaches the bivalve. Tapes, or else the 

 common edible cockle {Cardium edide). In one or 

 other of these molluscs the eggs or larvae commence 

 the first part of their life and develop into spherical 

 cysts. From the walls of these cysts other larvae are 

 formed. These leave the cockle (or Tapes) and if 

 fortunate eventually reach a mussel. 



They bore into the mussel and come to rest in its 

 tissues, and round them an epithelial sac of cells may 

 appear. Noav, if the mussel should be eaten by a 

 bird — the Scoter — the larvae reaching their final host 

 develop into the adult worm, and that is the end of 

 the story. 



If, however, the epithelial sac is formed and the 

 mussel does not fall a prey to some predaceous bird, 

 the trematode larva dies and round its remains the 

 epithelial sac secretes a mausoleum of pearl. A more 

 startling story could hardly be imagined, but most 

 of the life-cycles of animal parasites are extra- 

 ordinarily exciting. We may tabulate the stages as 

 follows : 



