PECULIAE AEEANGEMENT OF THE HEAD-PAPILLA. 507 



to this, but surrounding the greater part of the rolled-up process, as a 

 narrow space separates the latter in many places, as far as the last 

 segment, from the surrounding receptacle. 



I must confess that I did not at first recognise this peculiar 

 character of the head-process, which I know only in the bladder- worm 

 of the pig. I first understood it when I became convinced that the 

 formation of this outer cavity,. which isolates the head, results from an 

 annular fold which surrounds the base of the latter, and is distin- 

 guished essentially from the other folds of the internal space only in 

 this, that it develops to a greater depth, and grows round the whole 

 mass of the head-process like a bell. ^ Traces of this fold are present 

 in other bladder-worms. Thus, for example, in the Cysticercus from 

 the crow, described and figured above (p 343), the basal fold has only 

 to be deepened and expanded like a bell in order to isolate the head- 

 process, just as in the adult bladder-worm of the pig. 



It is obvious that the " outer cavity," in spite of its peculiar 

 character, represents an integral part of the head-cavity, for its wall 

 exhibits the same folds as we have seen to exist over the whole length 

 of the latter. The outer wall is indeed destitute of these folds, but 

 that is probably because of its slight thickness, and of the tension to 

 which it and the receptacle in general are subject. 



This unusual condition of the head-process deserves special notice, 

 if only because the early observers were largely mistaken as to the 

 position and attitude of the latter within the receptacle. This is par- 

 ticularly the case with Eobin's figures, reproduced by the elder van 

 Beneden^ and by Davaine,^ which are at least false, in so far as 

 they show the head-process rising from the base of the receptacle 

 ("vdsicule int&ieure," Dav. ; "membrane enveloppante," van Ben.). 

 If one press the tsenioid body of the bladder-worm out from the re- 

 ceptacle one may indeed see such appearances as Davaine figures 

 (after Eobin) ; but a closer examination shows how false are con- 

 clusions drawn ;from such cases. This fact alone is conclusive, that 

 the body protruding from the rupture shows the head, not in its 

 evaginated state, as Davaine represents it, but invaginated, as it 

 originally appeared, with cuticle and hooks internally.* Much more 

 correct and satisfactory are Steinbuch's representations, which, though 



^ The only one who has described this peculiar structure before is Moniez, but he 

 seems to me to have no oorrect idea of its true nature (loc. cit., p. 56). 



" "Zoologie m^dicale," ioc. «i. 



° Lnc. cit., p. 39, Figs. 3 to 6. 



* Kiiohenmeister has figured the circlet of hooks in a bladder-worm head pressed out 

 in this way, as the "Apex of the head (Vorderkopf) of Txnia solium" (" Parasiten," 

 first edition, tab. iii. , Fig. 8). 



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