12 THE ANIMAL PARASITES OF MAN 



ments (O. Fr. Miiller). It was the latter author also who recognised 

 the origin of the tapeworms (Schistocephalm, Ligula) found free in 

 the water. They originate from fishes which they quit spontaneously. 



Nevertheless, and in spite of the fact that van Doeveren and 

 Pallas correctly recognised the significance of the eggs in the trans- 

 mission of intestinal worms, these statements remained disregarded, 

 as did Abildgaard's observation, experimentally confirmed, that the 

 (immature) cestodes from the abdominal cavity of sticklebacks become 

 mature in the intestines of aquatic birds. However, at the end of 

 the eighteenth and the commencement of the nineteenth centuries, 

 and after helminthology had been raised to a special branch of study 

 by the successful results of the investigations of numerous authors, 

 '(Goeze, Bloch, Pallas, Miiller, Batsch, Rudolphi, Bremser), many 

 of whom experienced a "divine joy" in searching the intestines of 

 animals for helminthes, some authors reverted to generatio cequivoca, 

 without, however, entirely denying the existence of organs of genera- 

 tion and eggs. The fact that a few nematodes bore living progeny 

 a fact of which Goeze was already aware had no influence on 

 the erroneous opinion, as in such cases it was considered that the 

 young continued to develop beside the old forms ; there were also 

 many helminthes known that never developed sexual organs and 

 never produced eggs, and which therefore were referred to generatio 

 tequivoca. People were convinced that the intestinal mucous mem- 

 brane or an intestinal villus could transform itself into a worm, 

 either in a general morbid condition of the body, or in pathological 

 changes of a more local character. The appearance of helminthes 

 was even regarded as useful and as a means for the expulsion of 

 injurious matter. 



These views, firmly rooted and supported by such eminent 

 authorities as Rudolphi and Bremser, could not easily be overthrown. 

 First, a change took place in the knowledge of the trematodes. 

 In 1773 O. Fr, Miiller discovered the Cercarice living free in the 

 water ; he regarded them as independent creatures and gave them 

 the name that is still used at the present time. Nitszch, who also 

 minutely studied these creatures and who recognised the resemblance 

 of the anterior part of their bodies to a Distomum, did not, however, 

 arrive at a correct conclusion ; he regarded the combination 

 rather as that of a Distomum with a Vibrio, for which he mistook the 

 characteristic tail of the cercaria ; he also noticed the encystment 

 (transformation into the pupa) on foreign bodies of many species of 

 these creatures, but was of opinion that this process signified only 

 the termination of life. 



