SUBDIVISIONS OF PARASITIC WORMS. 267 



of life, a characteristic very important from a biological point of 

 view, but only of subordinate importance in determining natural 

 relationship. To base a classification on such physiological characters 

 is in this case all the more illegitimate, since we know that not a few 

 Helminths live for a longer or shorter part of their life in freedom. 



Also, the older zoologists have never scrupled to refer certain non- 

 parasitic forms, such as the Rlidbditis (Anguillula), to this group, 

 and to refer other really parasitic worms, such as Chcetogaster, 

 Hirudo, &c., to their place beside the true worms. 



When we further recall how many Helminths have an undeniable 

 relationship with free-living worms how the Trematodes, for ex- 

 ample, are, as we have seen (p. 105), closely allied to the Turbellaria 

 how the leech comes almost as a connecting link between the 

 Helminths and the higher worms we shall be sufficiently convinced 

 of the truth of the opinion staunchly upheld half a century ago by my 

 uncle, F. S. Leuckart x namely, that a class of Helminths has only 

 the rank and import of a faunistic group, and ought never to be in- 

 vested with a systematic unity. 



Since the time of Zeder and Kudolphi five distinct groups of 

 Helminths have been generally recognised, whether unified into a 

 special class or no viz., the thread-worms or Nematodes, the Acan- 

 thocephali, the flukes or Trematodes, the tape- worms or Cestodes, and 

 the bladder-worms or Cystici. The attempt of Diesing to form a sixth 

 order Acanthotheci of the so-called Pentostomata has only found 

 a limited support, and calls for no further notice, since we are de- 

 cidedly convinced (p. 14) that these animals, in spite of a certain 

 superficial resemblance to Helminths, have no more claim to be classed 

 as worms than have the Lern?eadcie (p. 92), which were referred to the 

 same class before v. Nordmann's investigations. Just as the latter 

 are Crustacea, as their larval stages indisputably prove, so are the 

 former to be regarded as peculiarly modified mites. 



Nor has it been possible for the five orders of Rudolph! to remain 

 unaltered, for experimental helminthology has established beyond 

 doubt the opinion of Dujardin, v. Siebold, van Beneden, and others, 

 that the bladder-worms were no independent species, but only stages 

 in the development of Cestodes. The order has thus vanished from 

 systematic zoology, and the number of groups of Helminths is thus 

 reduced to four, which have, according to all results, a reasonable 

 claim to be regarded as natural divisions. 



On closer inspection, however, it soon appears that they are not 

 equally closely united to one another. As we formerly showed 



1 " Versuch einer naturgemassen Eintheilung der Helminthen, nebst dem Entwurfe 

 einer Verwandschafts- und Stufenfolge der Thiere uberhaupt :" Heidelberg, 1827. 



