26 THEORY OF THE ORIGIN OF PARASITES. 



and yet no locality was left unsearched ; and gradually arose the 

 conviction that the statements of the free existence of intestinal worms 

 were, in the majority of cases, based upon a confusion of these worms 

 with others closely resembling them, and that in those instances (e.g., the 

 tape-worm found by Linne), where an intestinal worm appeared to 

 have been found living free, the discovery could not be interpreted in 

 the sense Linne supposed. 



A new theory took the place of the old one. Basing his opinion 

 upon the facts that the eggs of intestinal worms are expelled with the 

 faeces of the animal in which they live, sometimes enclosed in a por- 

 tion of the body of their parent, as in the Cestodes, and that they 

 remain unaltered for a long time in water, Pallas * put forward the 

 view that Entozoa agree with other animals in originating from eggs 

 which can be carried from one animal to another. " It cannot be 

 doubted," he says, " that the eggs of the Entozoa are scattered abroad 

 and undergo various changes without loss of vitality, and that im- 

 mediately they reach the body of a suitable animal, through the 

 medium of its food or drink, they grow into worms." Of course the 

 eggs in this way could only reach the alimentary canal ; but since the 

 Entozoa were found not only here, but also in other organs liver, 

 muscles, brain the only possible explanation was that the eggs 

 entered the blood-vessels from the intestine, and were carried " by 

 the blood stream " to those various and apparently inaccessible organs. 

 By the help of the blood-vessels, Pallas believed that the eggs 

 occasionally reached the body of the embryo before it was born ; in 

 this way intestinal worms could also be inherited by one host from 

 another. 



This was not, however, the first time that the theory of the 

 "inheritance of Entozoa" had been propounded. Even in the 

 days of Leeuwenhoek, Yallisnieri * had endeavoured to explain 

 the presence of Entozoa by supposing them to be transmitted 

 from parents to children ; and this hypothesis had many supporters, 

 including certain of his illustrious contemporaries (Hartsoeker, 

 Andry, &c.) and numerous later heiminthologists, as 0. F. Miiller, 3 

 Bloch, 4 and Goze. 6 On this hypothesis, the intestinal worms must 

 have originated in the way just indicated; they must have been 

 innate, or at least have been received by direct transference (for 

 instance by kissing or being suckled). Otherwise, a subsequent 



1 New nord. Beitrage, Bd. L, p. 43, Bd. ii., p. 80. 

 3 " Opere fisico med.. !) t. i., 1733. 



3 Naturforscher, Bd. xiv., 195, 1780. Neues Hamburger Magazin, Bd. xx., 1784. 



4 " Abhandlung von der Erzeugung der Eingeweidewurmer : " Berlin, p. 37, 1782. 

 6 " Versucheiner Naturgeschichte der Eingeweidewurmer: " Blankenburg, p. 4, &c. 



1782. 



