CHAPTER 4 



THE EFFECT OF PARASITES ON THE HOST 



If you join two lives, there is oft a scar 



Robert Browning 



*'r-r-iHE PEARLS of Britain," records Pliny, "be small, dim of colour and 

 J. nothing orient." But Julius Caesar openly admitted that the 

 breast plate which he dedicated to the Venus Genetrix was made of 

 English pearls. This may have been partof aCome-to-Britain campaign 

 designed to boost the Empire, but Pliny insinuates that the great 

 conqueror was mean and deliberately foisted second-rate pearls on the 

 Venus Mother. 



These pearls were found in the shells of fresh water mussels. Scot- 

 land enjoyed quite a flourishing pearl trade as late as the reign of 

 Charles II and the rivers Tay, Don and Spey were particularly famous 

 in this respect. It is said that one very large pearl from Wales is mounted 

 in the British Crown. If so it forms a fitting monument to the extreme 

 hazards of the trematode worm's life-cycle. 



Pearls in Britain to-day are found chiefly in the marine mussel 

 (Mytilus edulis) and not in the fresh water species {Unio and Anodonta), 

 They are usually formed round the body of a bird-parasite — a worm 

 which uses the mussel as an intermediate host and is found, in the adult 

 stage, in the reproductive organs of maritime ducks such as the eiders 

 and scoters. 



The mantle of the mollusc, a flap of skin which envelops the soft 

 part of the body, secretes a hard substance popularly known as mother- 

 of-pearl, with which it forms the lining of the shell. If the parasites 

 become accidentally attached to the outside of the mantle, they are 

 quickly enveloped in a covering of epithelial cells. These cells continue 

 to secrete and to envelop the worms in fine alternating concentric 



30 



