144 ANIMAL LIFE AND HUMAN PROGRESS 



fresh- water snails and in the tissues of these became " pupae." 

 Within these cysts there were found later young forms which 

 were to all intents immature flukes. 



Having, on morphological grounds, linked the free-swim- 

 ming cercaria with the fluke, he then proceeded to trace the 

 connecting link between the cercaria and the egg. To the 

 question, Whence comes the free-swimming cercaria ? he 

 found an answer already at hand in an observation made by 

 Bojanus, who had described cercariae swarming out from 

 " king's yellow worms," which occur in great numbers in the 

 interior of snails and which possess the characters of the 

 so-called genus to which the name Redia had been given. 

 These rediae or cylindrical worms of Bojanus resemble in 

 structure neither the flukes into which their progeny eventually 

 develop nor the young larvae which hatch out of the fluke 

 eggs. It was evident then that another stage of development 

 must intervene. Steenstrup found this link in some observa- 

 tions published several years previously by Siebold. In most 

 flukes the larva only develops within the egg after this has 

 been laid, but in a fluke called Monostomum mutabile, which 

 frequents the cranial cavities of certain water-birds, the eggs 

 develop while still within the parent. Siebold had remarked 

 that in this species the ciliated larva, developing within the 

 egg, always harboured another and totally dissimilar body 

 which he called the " necessary parasite." Steenstrup recog- 

 nised that this so-called parasite bore an undeniable resem- 

 blance to the " rediae " or worms of Bojanus, and thus finally 

 linked together " redia " and " cercaria " as different phases 

 of one life-cycle. 



In zoology, at any rate as applied to medicine, the inductive 

 method has its special perils, but of Steenstrup's remarkable 

 deductions it may be truly said that " Observation is blind 

 unless the eye is informed by knowledge. It is observation 

 ' loaded with inference ' that alone gives insight." 



The knowledge that the flukes require a molluscan inter- 

 mediary host in which to undergo their extraordinary meta- 

 morphosis, and that thereafter they may encyst in a second 

 intermediary before gaining their final or definitive host, 



