654 ORDINARY TAPE-WORMS (CYSTOIDEl). 



Among these there are first of all some species in which the head 

 has exactly the same position that we have described in the cystic 

 tape-worms as primary and normal. The head is completely invagi- 

 nated, so that the subsequent external surface forms the inner lining 

 of a pouch hanging down as a process in the axis of the bladder. 

 The hook-apparatus is found at the deepest part of the pouch. This 

 is not only the case in the young stage of Tcenia cucumerina (Fig. 338), 

 which in the structure of its posterior body is especially connected 

 with the Cysticercoids of the Cyclops; but so it is also in the so-called 

 Gyporhynchus of the tench (p. 364), which possesses a distinct caudal 

 bladder, although its head-end is sometimes protruded therefrom, and 

 used for creeping about in the host, just as do the larvae of the Tcenia 

 found in Cyclops. This slight difference is noticeable, that in the 

 Cysticercoids there is never the development of a vermiform body 

 intercalated as a long neck between head and bladder, as is seen so 

 frequently, and often so strikingly, in the true Cysticerci, especially in 

 C. fasciolaris. 



If our conception of the developmental history of the Cysticercoids 

 be correct (see p. 362), then the head has always at first the position 

 here described. The attaching apparatus is of course always formed 

 inside a bladder (the enlarged six-hooked embryo), the first rudiment 

 of the head appears as a hollow bud, which often at an early stage 

 exhibits an elevation at its base, which grows ever further outwards 

 until the head attains its final structure, and shows inside the dis- 

 tended sac-like neck exactly the subsequent position of the tape-worm 

 head. Since, so far as we know, it is especially the species with 

 extended rostellum, e.g., Cysticercus arionis (Fig. 336), which exhibit 

 this formation, we may suppose that the elevation of the head is 

 determined by the outgrowth of this structure, and that the position 

 of the head above noted has therefore only a secondary importance. 



As to the development of the Cysticercoids, I must refer to what 

 I have already said (pp. 360-368). This may again be specially 

 mentioned, that there are among them some forms which appear as 

 many -headed and racemose bladder-worms analogous to Ccenurus and 

 Echinococcus. 



As yet we know but few Cysticercoids, so few that one sometimes 

 still hears the doubt expressed whether all the hundreds of known 

 Tcenice have, indeed, passed through a Cysticercoid stage. We have 

 already referred to the difficulties attending the search for Cysticer- 

 coids (p. 379, Note), and can only further answer that until the 

 demonstration of a direct development our positive experimental results 

 must be held as decisive, and they all prove the existence of an inter- 

 mediate Cysticercoid stage. 



