18 ANNIE PORTER 



, VI. — Distribution of the Spirochaetes within 

 the Lamellibranch hosts. 



The crystalline style is the chief infected area in the Ostrea, 

 Anodonta, Mytilus and Sphaerium on wliich I hâve worked. 

 Examination of the entire aHmentary tract of t he above Lamel- 

 hbranchs has added but very httle to the observations made 

 by previous workers. 



Extremely few of the infected Anodons contained parasites 

 in the «flèche tricuspide)), and the portion of style nearesttoit 

 often contained practically no parasites. In fact, the heaviest 

 infection occurred in the posterior third of the style. This end 

 of the style is slightly more fluid than the anterior end, and 

 so may be more suitable for the parasites, ail of which seem 

 to prefer a fluid, slightly viscid médium. 



With regard to the zonal destribution of *S^. halbianii noted 

 by Perrin, I can say little. My expérience has been that oys- 

 ters obtained in England rarely contain styles, but the Spiro- 

 chaetes are free-swimming in the viscid gut contents which 

 contained style-substance in solution. 



The style of Aiiodonta cygnea varies. In a few, usually 

 uninfected spécimens, the style was striated. When the parasi- 

 tes were présent, as a rule they were f airly evenly distributed 

 throughout the breadth of the style, and did not concentrate 

 noticeably in the central, presumably slightly more fluid core, 

 but swam about indifferently, now approaching the edge, 

 now retreating to the centre of the style. 



With regard to portions of the digestive tract other than 

 the style and its sac, Spirochaetes are comparatively uncom- 

 mon. But on many occasions, I hâve inserted a pipette into 

 the mouth of a living oyster or Anodon whose valves were 

 fairly wide apart, and in the liquid so obtained Spirochaetes 

 were found. Extrême rapidity of manipulation is necessary, 

 otherwise the sudden closure of the shell both destroys the 



