OEIGIN OF INTESTINAL WORMS.,- 11 



of the presence of worms in insects, since here the origin of 

 the parasites is sufficiently obvious. Those who cannot make up 

 their minds to renounce the easy and convenient doctrine of 

 equivocal generation, may perhaps object, that the history I 

 have given of the propagation of the Mermis albicans stands 

 alone, and only makes an exception from the rule. To this I 

 answer, in the words of Goethe : " Nature goes her way, and that 

 which appears to us as the exception, is the rule." That this is 

 really the case in the present instance, is proved by recent 

 investigations into the natural history of the intestinal worms. 

 Since attention has been directed to their wanderings, more and 

 more facts have been daily brought to light, all tending to 

 show that the emigration and immigration of these parasites is a 

 much commoner and more widely extended occurrence than was 

 at first imagined. Habits, very similar to those which I have 

 just described in Mermis albicans, are also to be observed in 

 another thread-worm, the well-known Gordius aquaticus, which 

 has also been shown to live parasitically in the cavities of the 

 bodies of various insects, viz. : grasshoppers, terrestrial and 

 aquatic beetles, and in their larvae ; and to grow from a most 

 diminutive worm to one of several inches in length, which then 

 makes its way out, to attain to sexual maturity elsewhere, often 

 in the water. These facts were formerly wholly unknown, though 

 it must have long appeared surprising that this thread-worm, 

 which, on account of its form and colour has been compared 

 to a horse-hair, is, whenever met with in the water, of its full 

 size. But now that we know that the Gordius aquaticus, like 

 the Mermis albicans, enters in the embryo state into insects, 

 growing with them, and only quitting them when it has done 

 growing, the striking phenomenon I have mentioned is easily 

 accounted for. 



Just as, for the reasons already named, some kinds of para- 

 sites that have emigrated are never met with below a certain 

 size ; so, some kinds of parasites that have already made 

 their way into the interior of animals are not to be found 

 under a certain size, however often and carefully they may be 

 sought for, a circumstance which must certainly have been 

 noticed by many physicians and naturalists, without their having 

 paid further attention to it. It is now known that many para- 

 sites do not enter into the animals in which they are to pass 

 through their further stages of growth until they have attained a 



