SOUTHERN OYSTER DRILL AS A PREDATOR 



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Figure 8. — Redia with .volk platelets in posterior end of intestine (arrow), digestive gland, natural infection: Z. 



WH-AzB-E: X215. 



Microscopic examination of stained sections of 

 infected drills reveals : 



(1) Extensive destruction of the digestive 

 gland, due to redial ingestion of host tissue, which 

 may reduce intact tissue to 10-30 percent of the 

 area of a cross section of the gland ; (2) Compres- 

 sion of the remaining digestive gland tubules with 

 resultant obliteration of most, if not all, lumina 

 because of growtli of the large number of rediae; 

 (3) Basophilic inclusion granules of uncertain 

 significance in the cytoplasm of the large triangu- 

 lar cells of tlie digestive gland; (4) Amoeboid 

 cells, thought to be phagocytic blood cells, 

 grouped about some rediae in tlie digestive gland; 

 and (5) Gonadal damage whicli is directly related 

 in extent to severity of infection, inassi\e infec- 

 tions resulting in severe to total destruction due to 

 ingestion of host tissue by rediae (figs. 7-10). 



Naturally acquired Parorchis infections pro- 

 duce lasting damage. Forty-four infected drills 

 maintained in an aquarium with running sea 

 water and adequate food did not spawn during 

 an entire breeding season, and 26 of these failed to 

 spawn during a second breeding season (approxi- 

 mately 2 years in captivity) , the remainder having 

 been .sacrificed for various purposes. Similarly 

 maintained control snails spawned nonnally eacli 

 year. 



Duration of Infection 



It is not known precisely how long these infec- 

 tions last, but 26 naturally infected drills main- 

 tained in the laboratory for 2 years regularly 

 emitted cercariae wlien tested and all were heav- 

 ily infected when sacrificed for dissection or for 

 histological study. Thus, once established, the 



