January 22. 1903J 



NA TURE 



281 



■concentric layers of rod-like prisms, as in the brown or " black" 

 pearls of the Scotch river mussel, Margaritana margaritifera 

 <Fig. 1). 



By far the greater part of the mantle epithelium deposits the 

 nacre, and pearls which arise in this part of the mantle are the 

 typical nacreous ones, to which the great majority of the 

 marketable gems belong. Even the uncalcified substance of 

 the hinge ligament of the shell maybe represented in the pearl; 

 for example, ill the great Australian mother-of-pearl oyster, 

 Margaritifera maxima, Jameson, black leathery pearls are 

 ■sometimes found in the dorsal body-wall. 



I next proceeded to investigate the origin of the sac in 

 which the pearl arises, and also the nature and origin of the 

 "nucleus" which is so often to be found in the centre of the 

 pearl. In a great many molluscs, among which were several 

 of the pearl-oysters proper, the remains of trematodes were 

 found to form the nuclei, a discovery which confirmed the 

 -observations of Filippi, Mobius and others. In one or two 

 cases, however, other parasites played the same part. By con- 

 fining my attention to the common mussel, I proved that the 

 epithelial sac, which is all-important for pearl formalion, is first 

 formed around the live trematode which enters upon a resting 

 stage in the tissues of Mytilus. A similar sac, surrounding a 

 trematode, was found in an example of the Ceylon pearl- 

 oyster, Margaritifera vulgaris, Schumacher, which I ex- 

 amined. For the formation of the pearl, it is not necessary for 

 the trematode to persist as nucleus, for it often happens that 

 it migrates out of the sac ; but the sac, caused primarily by the 



parasite dies in this sac, a pearl is formed around its remains, 

 or, if it migrates to another part of the tissues, a pearl may be 

 developed in the empty sac. 



Although it was found impossible to secure live uninfected 

 material of the fina, host for experiment, it is almost certain that 

 the adult stage of the parasite is Distoma (Leucithodendrium) 

 somateriae, Lev., a worm which occurs in the intestine of the 

 eider duck, Somateria mollissima, and the black duck or scoter, 

 Oedemia nigra. Both these birds feed almost exclusively on 

 mussels. A number of scoters caught or shot in the immediate 

 vicinity of the Billiers pearl-beds were found swarming with 



this worm. Indeed, one example of the worm, in an immature 

 condition, quite indistinguishable from the resting stage which 

 occurs in Mytilus, was taken from the intestine of a scoter. 

 Our knowledge of the life-histories of other trematodes, or 

 "flukes" as they are popularly called, enables us to fill in the 

 life-history of this parasite with considerable detail. The worm 

 reaches maturity in the intestine of the scoter and eider, and 

 the eggs pass out with the faeces. These eggs, or possibly 

 "Miracidium " larvse derived from them, enter Tapes or the 

 cockle and there give rise to sporocysts, in which the young flukes 



Fig. i.— Section ground from a brown Scotch River Pearl, showing it to 

 be composed of the Prismatic Substance, X 25. 



•specific stimulation of the parasite, is essential to pearl produc- 

 tion. In Modiola modiolus, and probably in some other forms, 

 similar sacs are formed around Sporozoa. 



The next subject to be investigated was the origin and life- 

 history of the parasite that causes the pearl-sac. The common 

 mussel was found to be the most convenient species on which to 

 study this, and the pearl-bearing mussel-beds of Billiers, in 

 Brittany, and Piel, in the Barrow Channel, were selected as 

 suitable sites in which to begin the observations (Fig. 2). 



The parasite, like most trematodes, passes through a regular 

 cycle of three hosts, two of which are invertebrates and the third 

 a vertebrate. It arises in sporocysts in the "tapestry shell," 

 Tapes decussaius, and the cockle, Cardium edule, the former 

 acting as first host at Billiers, the latter at Piel, where Tapes 

 does not occur. The young tailless Cercarix" or trematode larv:e 

 leave the mother sporocysts in the first host and migrate into the 

 mussel. 



The transmission of the parasites from Tapes to Mytilus was 

 proved experimentally in a tank at the Brighton Aquarium. In 

 the mussel, the parasite enters into a resting stage, in the sub- 

 epidermal connective tissue, and gives rise to the epidermal sac 

 •or "epithelioma" in which the pearl arises (Fig. 3). If the 



,A 



A 



Fig. 3.— Diagrammatic section or part of the shell and mantle of Mytilus, 

 showing a trematode in its sac, a pearl in a similar sac and a secondarily 

 attached pearl ; 11., nacreous substance of the shell ; ef> ext., external 

 nacre-secreting epithelium of the mantle ; c.t., connective tissue ; s., sac 

 containing live trematode ; s\ sac containing pearl ; nu., nucleus of 

 pearl ; note also the sac of an " attached " pearl, which has become 

 continuous with the external epithelium of the mantle ; ».«./.. nucleus 

 of the atiached pearl ; e/>. int., internal ciliated epithelium of mantle, 

 which lines the branchial cavity. 



or Cercarise are formed. These larva;, unlike typical Cercarise, 

 are tailless, and when they escape from Tapes reach the mussel 

 chiefly by drifting with the tidal currents. On entering the 

 mussel, they pierce the body wall and settle down in the sub- 

 cutaneous tissues, in which they become surrounded by the 

 pearl-sacs. If the mussel lives long enough, pearls will be 

 formed in these sacs. If, on the other hand, a mussel contain- 

 ing these resting Cercaris is eaten by an eider duck or scoter, 

 the Cercarice develop into the mature worms, which produce 

 eggs, and the life cycle is repeated. 



As an economic result of these investigations, it would seem 



NO. I734, VOL. 67] 



