ORIGIN OF PARASITES II 



a free-living planaria (Dendrocoelnin lacteiun), the Oxyiiris venniciilaris 

 from free-living nematodes, and the Tcenia lata (i.e., Dibothriocephalns 

 latus) from a tapeworm (Schistocephaltts solidus) found free in the 

 water. Linnaeus' statements met with general approval. However, 

 we must bear in mind that at that time the number of helminthes 

 known was very small, and many of the forms that we have long 

 ago learned to differentiate as specific were then regarded as belonging 

 to one species. Linnaeus' statements were partly supported by similar 

 discoveries by other investigators, such as Unzer, and partly also by 

 the discovery of eggs in many helminthes. It was believed that the 

 eggs hatched in the outside world gave rise to free-living creatures, 

 and that these, after their introduction into the intestine, were trans- 

 formed into helminthes. By means of these eggs the old investigators 

 tried to explain the HEREDITARY TRANSMISSION of the intestinal 

 worms, which was universally believed until the commencement of 

 the last century. Some authors went so far as to regard the intestinal 

 worms as congenital or inherited ; they maintained the possibility 

 of direct transmission, as in suckling, and denied that the eggs 

 reaching the external world had anything to do with the propagation 

 of the parasites. 



The more minute comparison between the supposed free-living 

 stages of the helminthes and their adult forms, and the impossibility 

 of finding corresponding free forms for the ever-increasing number 

 of parasitic species, revealed the improbability of Linnaeus' statements 

 (O. Fr. Miiller). It \vas the latter author also who recognized the 

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

 water. They originate from fishes which they quit spontaneously. 



However, in spite of the fact that van Doeveren and Pallas 

 correctly recognized 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 became 

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

 the eighteenth and the commencement of the nineteenth centuries, 

 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 ccquivoca, 

 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 



