382 ON THE OlilGIN OF ENTOZOA 



explaining the difficulty, and successfully controverts the argu- 

 ments adduced in its favour. If the worms found in animals 

 were indeed to be met with in other situations, it would furnish 

 great support to that opinion which refers their origin to an 

 outward source, but those instances which have been adduced 

 admit of an easy solution. Linnaeus (System. Natur. Ed. xii.) 

 tells us that he has found the Dlstoma hepaticum, or liver- 

 fluke, in spring water; the Tcenia vulgaris rather small in 

 muddy springs ; the Ascaris vermicularis, or thread- worm, in 

 marshes, and in the roots of putrefying plants. It is evident 

 that this celebrated naturalist, whilst travelling in Gothland, 

 had confounded the features which characterize the Distoma 

 hepaticum and the Tcenia gasterostei solida, (which he has 

 often mistaken for Tcenia vulgaris,) with those of Planaria 

 lactea ; but what worm he could have mistaken for the 

 Ascaris vermicularis I am at a loss to conceive.*' 



As he had investigated but few Entozoa, and these only 

 incidentally, we cannot wonder at the vagueness and want of 

 precision in his descriptions. The fact of his having con- 

 founded the Tcenia of man with those of the Mammalia, and 

 having denied the existence of a head in all of them, is suf- 

 ficient proof that he had not submitted them to careful exami- 

 nation ; in the same way, therefore, he may have easily 

 mistaken extrinsic worms for human, the change in which he 

 had supposed to have taken place during the period of their 

 abode in the animal body. He has betrayed himself into a 

 similar error in the existence of the Lumbricus terrestris, or 

 common earth-worm, a variety of which he says he has met 

 with in the intestines ; he hastily concludes that this differs 

 from the Ascaris Lumhricoides, and improperly adds it to the 

 genus Lumbricus. 



Gadd informs us, that it had occurred to him in 1747 to find 

 the Tcenia articulata jlava, (characterized by two lateral 

 apertures,) and identical with that which infests man, in the 

 river Fennonia, which contains yellow ochre. As nothing fur- 



•* " Vermeil album molliusculum, cylindraceum, antice crassum subrostratum, 

 postice atteiuiatuni, pluribus abhinc annis, quod non dissimulandum, in aqua 

 palustri copiose reperi et microscopii ope pingi curavi, ab Ascaride vermiculari 

 tamen diversum, cum vero ejus descriptionem facere accuratiusque examen 

 instituere impeditus fuerim, nee postea unquam occurrerit, hoc seponere lubet. 

 Facile erit Ascaris vermicularis a periilustri a Linne in paludibus rcperta." — 

 Otto. F. Mullei: Fcrm. teircslr. el fluviaUl. Historia. Vol. I. Part II. p. 36. 



