ESCHRICHT UPON THE ORIGIN OF INTESTINAL WORMS. 33 



that Bothriocephalus (Schistocephalus) solidus and Ligula only at- 

 tained to full development when they passed from the body-cavity of 

 a fish to the intestine of a water-fowl, and stated that in all proba- 

 bility many other Helminths wander in a similar way from one 

 organ of their host to another. By these and other facts, Eschricht 

 arrived at the conclusion that the life-history of Entozoa must le 

 considered as analogous on the whole to that of the parasitic larvce of 

 ichneumon-flies and lot-flies, but that each instance demands a special 

 explanation, on account of the complexities possibly introduced. In 

 the meantime, however, no details could be given, but in all pro- 

 bability the various asexual parasites so frequently met with 

 encysted in the muscles and connective tissue, such as bladder- 

 worms, Filaria (including Trichina spiralis), and Echinorhynchus the 

 latter being occasionally during the summer found in thousands in 

 the flesh of fishes must be regarded as immature forms, retaining 

 their primitive larval situation. 



We shall find later on that Eschricht had hit upon the truth in 

 pointing out that change of place and of form were the most im- 

 portant facts in the life-history of parasites. But there were none 

 of the necessary details forthcoming to prove his explanation, and 

 it appears, therefore, in spite of his statements and the support 

 which Valentin's 1 observations lent to them, that the majority of 

 helminthologists continued to uphold the old theory of the spontaneous 

 generation of intestinal worms. 2 



But gradually more and more light was thrown upon the obscurity 

 which enveloped the whole subject of parasitic worms. Shortly after 

 the publication of Eschricht's researches appeared Steenstrup's famous 

 work upon the alternation of generations, which rendered intelligible 

 so many facts in the developmental history of the lower animals that 

 had been previously but incompletely appreciated. The discoveries 

 and arguments brought forward by Steenstrup proved conclusively 

 that there are animals whose descendants in the second or third gene- 

 ration return to the original form of the sexual animal, and that 

 numerous intestinal worms belong to this class. 



The proof of alternation of generations was most completely 

 obtained from the Trematodes, 3 and quite simply, for Steenstrup con- 

 nected their life-history with the above-mentioned Cercariae. By 

 discovering that these latter, in spite of their independent origin, were 

 really larval Trematodes, he determined the fate of a large group of 



1 Repertorium /. Anat. u. Physiol., Bd. vi., p. 50, 1841. 



2 Creplin, Art. " Enthelminthologie " in Ersch u. Gruber's "Allgem. Encyclop. 

 d. Wiss.," Leipzig, 1818-1846, Bd. xxxv. 



3 "Ueber den Generationswechsel : " Copenhagen, p. 50, 1842. Translation by 

 Ray Society, London, 1845. 



C 



