I. INTRODUCTION. 



While securing the eggs of Littorina rudis I found in the brood-sac of the female 

 a small ciliate parasitic upon the ova therein contained. A discussion of the struc- 

 ture and relationships of this parasite, an undescribed species, is the purpose of the 

 present paper. 



My thanks are due Dr. Alexander Agassiz for the opportunity of obtaining the 

 material upon which this paper is based and for the privilege of beginning this study 

 in his marine laboratory at Newport in the summers of 1892, '93, and '94. 



II. METHODS. 



The mollusks were removed from the broken shell and the brood-sac was dis- 

 sected out and opened. The ova and parasites together were then washed out into 

 watch-glasses by a spray from a fine pipette and the whole killed in Flemming's 

 fluid, in Merkel's chromo-platinic mixture, or in picro-sulphuric acid. The unstained 

 Flemming material gave excellent results for all details of structure, except the 

 micronucleus. Ranvier's picrocarmine and Benda's iron-hsematoxylin were used 

 for staining and 5% carbonate of soda for the demonstration of cilia, striae, and sur- 

 face markings after the manner suggested by Schewiakoff ('96). Some Flemming 

 material placed in 5% carbonate of soda for two days and afterwards stained in 

 picrocarmine gave the best demonstration of the micronucleus. The parasites 

 were isolated by means of capillary pipettes either before or after staining. Beech- 

 wood creasote was used as a clearing agent, and thin xylol-damar for mounting. It 

 was possible by pushing the cover-glass to orient the specimen in any desired position. 



III. PROTOPHRYA OVICOLA gen. nov., sp. nov. 



This species was found in the brood-sac of Littorina rudis Don. More than 

 50% of the females of this gasteropod were thus parasitized, and often by great numbers 

 of these ciliates. The brood-sac is an expansion of the distal portion of the oviduct 

 with richly glandular walls in whose folds the eggs are retained during their devel- 

 opment. L. rudis is ovoviviparous and the eggs are somewhat richly laden with 



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