148 THE BIOLOGY OF THE SEA-SHORE 



injury is due to the presence of sporocysts and rediae, 

 encysted cercariae seeming to do little or no harm to the host 

 even when present in great numbers. The adult flukes are 

 mostly to be found in various fishes and sea-birds, which 

 feed upon the molluscs containing the intermediate stages 

 above mentioned (Lebour, op. cit.). Jameson and Nicol 

 (1913) state that no fewer than sixteen distinct species of 

 Trematodes are known from the Scoter Duck {(Edemia 

 nigra). 



More than academic interest attaches to the life-histories 

 of these flukes, since the cercariae are often responsible for 

 the formation of pearls. Jameson pointed out in 1902 that 

 the agent in forming pearls in the common edible mussel 

 {Mytilus edulis) is the larval stage of a Trematode, possibly 

 Leucithodendrium somaterice {Gymnophallus oedemice), which 

 lives in the Scoter Duck. The cercaria reaches the space 

 between the mantle and the shell and causes a portion of the 

 epithelium of the mantle to become separated off as a circular 

 sac. The cells forming the sac then secrete an organic 

 substance known as conchiolin and also calcareous salts, 

 which are deposited in concentric layers around the remains 

 of the parasite. The precise method of pearl formation 

 in different molluscs is still a matter of discussion. It 

 is quite certain, however, that true pearls are by no 

 means always due to the presence of parasites, but may 

 originate from the inclusion of foreign bodies such as sand- 

 grains or from some internal cause. A good account of 

 the various theories of pearl formation will be found in 

 Dakin (19 13), who summarises the most important investiga- 

 tions up to that date. 



In view of the great number of Trematodes whose 

 existences are dependent on those of shore animals the 

 comparative rareness of larval tapeworms in shore inverte- 

 brates is a matter for surprise. Many fishes, of course, 

 harbour tapeworms ; the skates and rays, for instance, are 

 particularly subject to the attentions of Cestodes, but the 

 intermediate stages are usually in other fish. Cysticercoids 

 have been obtained from various molluscs, e.g. Solen vagina 



