GORDIUS AND ECHINORHYNCHUS. 113 



thread-worm which differs from the real and typical Nematodes in 

 many respects, among which may be mentioned the atrophy of the 

 intestine, and the terminal position of the male and female sexual 

 apertures, characters which approximate it to the Acanthocephalidse. 

 This is, however, only an additional reason for laying greater stress 

 upon it, since we have every reason to consider the Acanthocephalidre 

 as yet more modified forms. 



The changes which lead the embryos of Gordius to their ultimate 

 structure are unfortunately yet unknown to us. This fact is the more 

 to be regretted, as they may acquaint us with relations which would 

 bring the strange and in many ways remarkable metamorphosis of the 

 EchinorJiynchi* nearer to the usual process of development than has 

 hitherto been the case. In the meantime, in considering their re- 

 lationship, we can lay only slight stress upon these peculiarities, for 

 we are well aware that the developmental history often pursues various 

 courses even in closely related animals ; in one case it may be direct, 

 and hasten rapidly to its goal, in another, it may reach its conclusion 

 by a circuitous route, passing through metamorphosis and alterna- 

 tion of generations. The course of development of the Echinorhynchus 

 is merely a metamorphosis a metamorphosis, too, than which nothing 

 more thorough and complete could be imagined, since in its course 

 almost everything that the fully developed worm possesses is formed 

 anew out of the older structures. 



After the foregoing account, the reader may decide for himself 

 whether, and how far, I have succeeded in discovering the relation- 

 ships of the Helminths, and in proving that they have originated 

 from free-living worms by adaptation to a parasitic mode of exist- 

 ence. But even suppose the matters just discussed were proved 

 facts, and not mere possibilities, even then much in the life-history of 

 these animals would remain problematical. We could only conclude 

 from this that a worm is capable of exchanging a free life for a 

 parasitic one, and of adapting itself in structure and mode of life to 

 such altered conditions. Instead of a free creature, the worm be- 

 comes a parasite, which departs, more or less, from its original form 

 according to circumstances. It now attains sexual maturity in the 

 interior of its host, instead of, as formerly, in the free state. It pro- 

 pagates, and generally, in consequence of the favourable circumstances 

 of nutrition, has usually a very numerous progeny, which pass to 

 the exterior, and perhaps for a time live freely, but finally develop 

 into sexually mature parasites. 



This is so in many instances, not only in stationary parasites, but 

 also in many Entozoa, though very seldom ; for, as a rule, the first host 



1 See Vol. II. 

 H 



