ON TREMATODE PARASITES. 53 



with carmine and broadly edged with yellow or orange, the 

 yellow bands occupying the upper and lower fourths of the 

 caudal. Female paler, more translucent, at times pinkish ; fins 

 white, doi'sal and anal dotted with carmine. 



Total length 42 mm. 



Several specimens. 



Allied to F. gardneri Blgr. 



This species will be figured in the forthcoming third volume of 

 the British Museum Catalogue of African Freshwater Fishes. 



EXPLANATION OF' PLATE IIL 



Fi<T. 1. Barhus spiirrelli. 



2. Sarilius macrostoma. 



2 a. ,, „ Head from above. Natural size. 



4. On some Parasites of the Scoter Duck {QlJdemia nigra), 

 and their Relation to the Pearl-inducing Trematode in 

 the Edible Mussel {Mytilus eduUs). By H. Lyster 

 Jameson, M.A., D.Sc, Ph.D., and William Nicoll. 

 M.A., D.Sc, M.D. 



[Received October 18, 1912: Read November 12, 1912.] 



(Text-figures 11 & 12.) 



Index. 



Sjstematic : Page 



Gymnophallus oedeniUe, noiii. iiov 67 



G. affinis, sp. n 58 



G. macroportis, sp. n 60 



G. ovopleiius, sp. n 62 



Of tlie many questions connected with the formation of pearls 

 in the common Edible Mussel (^Mytilus edulis) the identity and 

 life-history of the pearl-inducing organism is one of the most 

 important. It was shown ten years ago by Jameson (1902) that 

 the agent in this particular case is the larva of a parasitic 

 Trematode, which, instead of secreting a cyst of its own, as is 

 usual with such larvee, stimulates the mussel to form around it a 

 sac of epidermal cells. These cells possess the same physiological 

 properties as the outer shell-secreting epidermis, and eventually, 

 on the death of the Ti'ematode larva, secrete conchyolin and 

 calcareous salts, which, deposited in concentric layeis around the 

 remains of the worm, become the pearl. Attempts were made by 

 Jameson to trace the life-history of this pai-asite, but the diifi- 

 culties in the way of working out the complete life-cycle of 

 digenetic Trematodes are considerable, and the results obtained 

 by him in 1902 have not been accepted as entirely conclusive. 



With regard to the parasite in Mytilus, the two main questions 

 to be solved were : (1) Whence does it come ? and (2) Whither does 



