THE ORIGIN OF THE PEARL 25 



size, they form nodules in the mantle and cause the shell 

 to be built out around them, so that they leave pits in the 

 nacre similar to pearls. The marginal cyst distomid is 

 of especial interest on account of its connection with 

 pearl formation. Suspicion that this distomid concerned 

 the formation of the pearls was aroused when the objects 

 were first found, as their form and position in the mantle 

 were especially suggestive. This suspicion was intensi- 

 fied by frequently finding small round pearls in the 

 mantle of mussels, closely associated with the cysts. 

 Finally one of these pearls was examined and a cyst was 

 found to be the nucleus of the pearl. 



The marginal cyst distomid is very widely distributed, 

 being found in the Mississippi, Illinois, and many other 

 rivers. 



4. The Distomid of O shorn. Anadonta was often 

 affected with small white sporocysts, which covered the 

 outside surface of the mantle next to the nacre. The 

 nacre of these mussels was often raised into a number of 

 sharp, pearly blisters conforming with the size and shape 

 of the sporocysts, and indicate that the sporocysts had 

 been actually covered with nacre. Some sporocysts were 

 carefully examined. The skin of the sporocysts was 

 transparent enough to reveal, in the interior, the cercarial. 

 It has been learned that the cercarial later come out from 

 their sporocysts and pass through various changes until 

 it has the typical form of a Distomum, when it moves up 

 into the umbonal region of the mussel and acquires a 

 pinkish color. There it remains and stains the nacre a 

 salmon color. The distomid of Osborn is said to form 

 dorsal baroque pearls in the anadonta. 



