188 ANIMAL PARASITES AND MESSMATES. 



Thus it was considered by Lamarck that parasitical 

 worms are only fomid in the bodies of animals, and are 

 actually produced there. 



Can it be believed that such ideas were put forward 

 by zoologists of the highest merit? and ought we to feel 

 surprised that the theory of spontaneous generation was 

 so long taught in the physiological schools ? 



A book published in 1859 was entitled, *' Hetero- 

 genesis, or a Treatise on Spontaneous Generation." The 

 author gives the clue to the origin of his errors in the 

 second line of his preface, in which he says : " When, 

 hy meditation, it was evident to me that spontaneous 

 generation was one of the means employed by matter for 

 the reproduction of living beings." .... According to 

 this philosopher, science is, therefore, not the general- 

 ization of facts, but these facts must serve to prop up 

 the theories or hypotheses invented in the silence of the 

 study. This passage of his work shows us that he was no 

 more able to yield to the evidence of experiments made 

 on worms, than to those of Pasteur on the infusoria. 



It may be related to the honour of the illustrious 

 Baer, that, from the year 1817, during his stay at 

 K5nigsberg, he took up arms against this hypothesis, 

 and never ceased to combat it, till evidence succeeded in 

 opening the eyes of the most obstinate. 



The worms which present the most remarkable 

 phenomena of transformations, accompanied by metamor- 

 phoses, are the Distomians and Cestodes, flat worms, 

 which we will consider in the first place. 



Trematode worms include a certain number of large 

 and beautiful parasites which scarcely undergo any 

 change, and are found only on the skin and the gills of 



