Abstract
The advantage of using laboratory-reared animals in the elucidation of trematode life histories is now generally recognized (Palombi, 1938; Stunkard, 1938, etc.). Further researches seem to emphasize this point. No matter how carefully controls are examined and no matter how precise is the morphological comparison between cercaria, metacercaria and adult fluke, the fact that the intermediate and final hosts have been exposed to previous infection in the wild—particularly by closely related species of worms—immediately detracts from the value of the experiments.