THE CESTOIDEA 



139 



become scattered amongst the hair of the dog, and are swallowed 

 by its parasites, the flea (Pulex serraticeps) or the louse (Trichodedes 

 canis), in the tissues of which the proscolex elongates ; an excentric 

 cavity (" primitive lacuna ") arises by the liquefaction and de- 

 generation of the internal cells, and the proscolex becomes pear- 

 shaped, the hooks being at the narrow end (Fig. XXIX.). It is 

 possible now to distinguish a " body " and " tail " (the " cystozooid " 

 and " acanthozooid " of Villot). In the former a pair of excretory 

 tubules make their appearance, which open by a median bladder 

 and pore at the base of the tail. At the anterior end of the 

 " body " the forecasts of the rostellum and the four suckers suc- 

 cessively appear as cellular thickenings, followed by muscular differ- 

 entiation, in the wall of the body ; they mark out a " fore body " 

 from a " hind body " ; the tail, meanwhile, becomes constricted 

 from the latter. The rostellum becomes pitted, 1 as also does each 



FIG. XXX. 



The cysticercoid of Hymenolepis 

 murina from the villi of the mouse's 

 intestine (after Grassi and Rovelli). 

 The tail is greatly reduced. 



FIG. XXXI. 



Cysticercoid of Dicranotaenia cu- 

 neata which occurs in the earthworm 

 Allolobophora faetida. The tail and 

 hind body have coalesced, the hook- 

 lets now appear on the wall of the 

 small bladder. 



sucker rudiment, and the whole fore body now becomes invaginated 

 into the hind body. Meanwhile, the tail becomes larger and 

 bladder-like, and in this way the hooks become separated, one pair 

 remaining subterminal 



The organism, before the invagination, resembles an immature 

 Caryophyllaeus, or even a Trematode cercaria, in which as here the 

 tail is a temporary larval organ. 



The tail drops off when the intermediate host is swallowed by 

 the dog ; the " cystozooid," which is set free in the intestine of the 

 latter, is a more or less spherical sac containing the invaginated head 

 or " scolex," which nearly fills the cavity of the sac (Fig. XXIX. 6). 

 Such a metacestode is known as a " cysticercoid " or parenchymatous 

 bladder-worm. The head now evaginates, and its base commences 

 to elongate and to become segmented to give rise to the strobila. 

 The wall of the cysticercoid and of the following metacestodes is 

 muscular, and contains lime cells and ramifications of the excretory 



- Grass! homologises this pit with the buccal cavity and pharynx of Trematoda. 



