1 40 THE CESTOIDEA 



and nervous systems, which are absolutely continuous with the 

 systems in the scolex. In this instance it is evident that there is a 

 gradual metamorphosis of the proscolex into the scolex and neck 

 of the strobila. 



In H. murina the tail of the onchosphere is so reduced as to form 

 part of the hind body, which envelopes the fore body or scolex, 

 and the aperture by which the latter has been invaginated closes 

 (Fig. XXX.). 



A large number of other cases are known in which the in- 

 vaginated scolex occupies almost the entire cavity (Fig. XXXI. ). 

 In some other tapeworms this cavity is larger, but filled with a 

 loose tissue, as in the case of Tetrarhynchus, Calliobothrium, etc. But 

 in the tapeworms of most mammals, e.g. T. solium, this cavity 

 becomes very much more extensive, since the six-hooked embryo 



Fia. XXXIL The life-history of a Cysticercus (Bladder- worm, or hydatid). a, scolex ; 

 5, fore body ; c, hind body+tail, caudal vesicle, or bladder. 



1. The solid proscolex. 



2. The primitive lacima has formed as before, but the organism is greatly distended by the 

 accumulation of fluid in this cavity. The body and tail are no longer distinguished (cf. Fig. 

 XXIX.), and the booklets have become cattered. Excretory tubules have made their appear- 

 ance in the wall of the " bladder." 



3. A further stage of this hydropic distention. An imagination of the wall of the bladder 

 has arisen in the "fore body." 



4. The invagination proceeds; the suckers and rostellum arise by evaginations at the 

 bottom of the tubular fore body, which hangs freely in the cavity of the cysticercus. 



6. The scolex is commencing to evaginate ; it rises up from the bottom of the " receptacu- 

 lum scolecis " (or fore body), and at the same time the sides of the latter commence to evaginate. 



6. Eversion is complete the sides of the fore body folded together, fuse to form the neck. 



7 to 10 refer to Taenia serrata (after Leuckart). 



7. The entire bladder-worm, Cysticercus pisiformis, from the rabbit, with scolex everted ; 

 the excretory network in the wall of tlie bladder or caudal vesicle is shown. 



8. The "bladder" (hind body+tail) has undergone digestion on reaching the stomach of 

 the dog. 



9. A further stage, in which the fore body is also destroyed, leaving only the " scolex." 



10.--The scolex by elongation and " segmentation " of its neck has given rise to a young 

 strobila. 



swells up by absorption of fluid l while encysted in the intermediate 

 host (Fig. XXXIL). The hooks become scattered, and no " tail " is 

 differentiated. Here, as in H. murina, the acanthozooid is not dis- 

 tinguishable from the cystozooid. Then there arises at one end a 

 small tubular invagination ( = the " fore body "), at the bottom of 

 which the forecasts of the rostellum and suckers are formed. The 

 " fore body " is here known as the " receptacle of the head," which 

 in some cases, as C. pisiformis, may be very long. When this 

 " bladder-worm " or cysticercus is swallowed by the final host the 

 head (scolex) evaginates, and the original bladder ( = " hind body + 

 tail ") which is now known as the " caudal vesicle," is digested, and 

 the scolex gives rise to the strobila as before (Fig. XXXIL). 



This cysticercus leads on to another form of metacestode, with 

 a still larger bladder, from the wall of which a great number of 



1 The fluid contained in a cysticercus is a weak saline solution containing '2 tc '3 

 per cent of albuminoids, possibly nutritive. 



