84 LIFE-HISTOEY OF PAKASITES. 



Our remarks, moreover, are only directed against the practical 

 application of the degeneration theory, and iiot to an equal extent 

 against its theoretical truth. Degeneration per se is quite as possible 

 among Helminths as any malformation, which is the result of an 



unusual or insufficient combination of the 

 causes of development. In this way are to 

 be regarded certain varieties in the form of 

 Helminths, as, for example, the so-called 

 JEchinococcus muUilocularis or Gysticercus ra- 

 cemosus'^ (Fig. 59), considered even quite 

 Fig. 59.-Oysticercus raeemosus recently to be " degenerated " (that is to 

 (after von Siebold). ^ay, pathologically altered) forms. We are, 

 therefore, in principle brought back to von Siebold's standpoint, or 

 even to that of PaUas (1767) and Hartmann (1685), who were the 

 forerunners of von Siebold in this theory,' but, nevertheless, we cannot 

 agree with the applications of that theory, and the conclusions that 

 were drawn from it. 



This matter, which has been briefly touched upon, leads us 

 naturally to an inquiry into the conditions of development among 

 Helminths, or, in other words, the influence which the environment 

 exercises upon their development. 



An Entozoon, as we know, only develops when its host can fulfil 

 its needs. What happens, then, if it cannot ? It has been already 

 said that a parasite under these circumstances perishes ; but the con- 

 ditions which give rise to this decay, and the time that it takes, are 

 subject to great variations. In the next place, we may mention that 

 the conditions which render development possible are remarkably 

 restricted, so that circumstances of the most varied kind come into 

 play. It is, for example, a well-known fact that the famous Coenurus 

 only develops in lambs. It has been proved experimentally that the 

 older the lamb becomes the more difficult it is to rear the worms ia 

 its body. After feeding the animal with the young parasites, it can 

 be stated with almost mathematical accuracy when it will begui to 

 show the effects of the infection ; while it can be stated with the same 

 probability that an older sheep will continue in good health though 

 infected with an equal number of parasites. These facts are not 

 peculiar to Coenurus, but common among bladder-worms, though 



^ See for a description of this peculiar form Heller in Ziemssen's ' ' Handb. d. sp. 

 Path. u. Ther.," Bd. iii., p. 334 (Eng. transl., "Cyclop. Pract. Med.," vol. iii., p. 598, 

 London, 1875), and Marchand, Virchow's Archivf. pathol. Anat, Bd. Ixxv., p. 104, 1879. 

 Numerous other observations have been made on the frequently irregular forms of the 

 bladder- worm of the brain in man by von Siebold, Krabbe, and others. 



^ See the historical introduction in my " BlasenbandwUrmer," pp. 11-13 : Giessen, 

 1856. 



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