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destructive to the sheep ; in many cases whole flocks having been 

 destroyed by it. Similar forms are met with among all animals 

 in the four higher classes, and among the lower the mollusca 

 are equally infested by them. Several of these trematoda when 

 young are not connected with any organ, but enjoy the power of 

 free locomotion in water externally to the animal, which in their 

 future state as entozoa they exist. In this condition they are 

 provided with a locomotive organ, by means of which the animal 

 propels itself through the water like a tadpole. In this larvae 

 state the trematoda are known to naturalists under the generic 

 name of cercaria. The swimming movement of this Cercaria 

 is very characteristic ; in performing it, the animal curves its 

 body together into a ball by burying the head near the caudal 

 extremity, and then elongating itself in sigmoid flextures, so 

 that it presents the appearance of the figure 8. In this way they 

 may be seen swarming about water-snails in great profusion. 

 The reason for their keeping in the neighbourhood of these 

 snails, is evident on placing them in water with these mollusks. 

 After swimming about them for some time, they affix themselves 

 by means of their suckers to the slimy integument, and finally 

 work themselves into the snail, loosing at the same time their 

 caudal appendage. In this pupae form they often may be found 

 in the mantle of the snail in great abundance. How long they 

 remain in this pupag state is unknown. Steenstrup states that 

 they do so not only for many weeks, but also many months. 

 He states his observations were made on them in the months of 

 July and August. In January following he noticed the change, 

 in the animals escaping from their pupae cases and assuming the 

 Distomata form (a 8.) 



The further advance of this fluke to a fully-developed animal 

 as we know perhaps it may be determined from consideration 

 of the other Distomata and Monostomata, in which the first 

 stage of development has been shown by the observations of 

 Siebold and others, our present information would lead us to 

 conclude that it deposits ova, from which, either within the 

 material body or without it, oval-shaped young proceed, which 

 move about briskly in the fluid contained in the interior of the 

 snail, or in the surrounding water, and bear no resemblance to 

 their parent. In what way this progeny is transformed into a 

 fluke, or as we know into a Cercaria, is as yet an unexplained 

 mystery ; but this change can and does take place throughout 

 only by the intervention of several generations, may be assumed 

 as beyond doubt ; and if we examine attentively what is taught 

 us by Bojanus. Baers, and Siebold respecting the development 

 of the Cercaria, and interpret the phenomena thus presented in 

 a natural way, we may to a certain extent penetrate the obscurity 

 which involves the solution of this enigma. Whence comes 

 then the free swimming Cercaria .-' This question is answered 

 by the observation of Bojanus, who states that this species is the 

 same with those he saw swarming out from the "King's yelloiu 



