378 THE DEVELOPMENT OF CESTODES. 



body-cavity. 1 The strap-worms (Ligula} which inhabit the body- 

 cavity of our freshwater fish attain a still larger size, and also ad- 

 vance further in development, since they form their sexual organs 

 while still in their intermediate host. The same is true of the 

 Schistocephalus of the stickleback, which is, upon the whole, very 

 nearly allied to the strap-worms, and is indeed only distinguished 

 from them in this, that the tape-like body is externally jointed, and 

 that this segmentation takes place during the larval state. 2 



It will thus be understood that the hosts of the Ligulidae are 

 endangered in a high degree by the growth of their parasites. In 

 nearly every case their parasitism results in a chronic peritonitis, to 

 which large numbers of fish fall victims. There are even distinct 

 instances of epidemics being caused by these worms, and especially 

 by Ligula* 



It might, however, be doubted whether the Ligulidae live from 

 the first in the body-cavity. At any rate one finds not unfrequently 

 in the liver of their host Helminth -capsules, enclosing a young, still 

 undifferentiated, Cestode of simple shape, which might quite well be 

 the larval form of Ligula or Schistocephalus. In this case trans- 

 ference into the body-cavity would of course soon follow, since only 

 a few weeks after infection with ciliated embryos (apparently intro- 

 duced through the intestine, and not through the skin) Donnadieu * 

 found young strap -worms in the body-cavity. Although some of 

 them were only from six to seven millimetres long, they possessed 

 essentially their subsequent structure. 



So far as we know, there is no other Cestode besides Archigetes 

 which completes its development without interruption or change of 

 host, or, what is the same thing, becomes sexually mature in its first 

 host. 5 And we can indeed hardly count Archigetes an exception, 



1 From the manifold nature of the hosts alone it is very improbable that all these 

 worms belong to the same species (see p. 12). 



9 It is true that when we remember that the true Ligulidae, and especially the still 

 small young specimens, also exhibit distinct traces of segmentation, this difference becomes 

 only one of degree. 



* See Donnadieu, " Contributions a 1'histoire de la Ligule," J&urn. anat. et phy&iol., p. 

 321, 1877. 



* Loc. cit., p. 452. 



* Even on this ground the assertion of Knoch that, in the human intestinal canal, 

 Boihriocephalus latus develops from the six-hooked embryo (even from the undeveloped 

 egg ! ) directly into the later tape-worm, has only slight probability. When describing 

 the worm more specially we shall afterwards return to the views and experiments of this 

 author (" Naturgesch. des breiten Bandwurmes," Mem. Acad. St. Petersb., t. v., No. 5, 

 1862), but it may now be mentioned that the experiments on which Knoch founds his 

 assertion have by no means the cogency which he fancies. [Braun of Dorpat has 

 shown experimentally, as we shall see hereafter, that Bothriocephalus latus has an inter- 

 mediate host in its young stage, just like other Dibothria, and that its larva is transferred 



