Focal aggregates of leucocytes were present in the intestinal epi- 

 thelium, or adjacent to it, in 14.2% (8 of 127) of the gut tissues 

 examined. One case involved a large, loose aggregation of leucocytes 

 in the leydig tissue beneath the basement membrane. Another involved 

 small clumps of leucocytes between the columnar epithelial cells. Like 

 the general leucocytosis , this condition was difficult to judge. Leuco- 

 cytes are normally present in the epithelium, but more or less scattered 

 about. These clumps could be normal phagocytosis, although no foreign 

 matter was ever observed in such clumps. The intestinal epithelium 

 containing the above clumps appeared otherwise very healthy. 



Focal necrotic areas were present in the gut epithelium of 3 (2.3%) 

 oysters examined. In two incidences, the gastric shield was involved. 

 This condition was characterized by a breakdown of the structure of the 

 gastric shield and/or epithelium, a concentration of debris at the 

 affected area, and leucocytic inflammation of the gastric shield and/or 

 epithelium. 



Ciliates were present in the gut lumen of one C. gigas. These 

 ciliates were the same type as described above in the digestive gland. 



The plasmodial stage of an unidentified sporozoan was noted in the 

 epithelium of a single C. gigas. The Plasmodium was amoeboid in appear- 

 ance with several nuclei. The gut otherwise appeared in very good 

 condition. 



Copepods were present in the stomach of 15.7% (20 of 127) of the 

 specimens examined. No oysters from Aber Wrac'h bore copepods. None of 

 the copepods observed were being phagocytized as was the case in the 

 digestive gland. Up to three copepods were observed in some sections. 



4. Gonad . - Gonadal tissues of 130 C. gigas were examined. Gener- 

 ally, two tissue samples were taken from each specimen (anterior and 

 psoterior visceral mass). 



Gonadal tissues from C. gigas were a very difficult tissue type to 

 assess for non-symbiotic pathologies. Possible histopathologies were 

 noted in 38% (9 of 130) of the gonadal tissues examined. There were 50 

 incidences of the three non-symbiotic pathology (?) types and one symbi- 

 otic pathology discussed below. Only six of the 50 conditions were of 

 an apparent symbiotic nature. 



Half (36 of 71) of the female gonadal tissues examined exhibited 

 moderate to heavy aggregations, both focal and general, of eosinophilic 

 leucocytes. This presented a perplexing problem in determining if this 



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