142 THE CESTOIDEA 



Just as the development of the head is delayed in these last 

 forms, so apparently there is a delay in the differentiation of .the 

 fore body, for in Monocercus we have a bladder which "buds" 

 off a secondary cyst within, from which the head is developed as 

 before (Fig. XXXIII. 1, 2, 3) ; while in Polycercus occurring 

 in Lumbricids, and which is the metacestode of T. nilotica, etc. 

 several such secondary cysts are formed, each with a head within it. 



But this formation of secondary cysts reaches a maximum in 

 " Echinococcus," where the primary bladder attains an enormous 

 size, and from its walls numerous secondary bladders are formed, 

 which drop off into the primary cavity (XXXIII. 5) ; from each of 

 these secondary (and even tertiary) bladders a great number of heads 

 are formed, each of which, when the bladders are swallowed, will 

 develop into a strobila. 



Fio. XXXIII. Asexual reproduction by the Bladder-worm. 



1 to 8. Polycercus, from the coelom of an earthworm ; it is the bladder stage of Taenia 

 nilotica, out of Cursorius eurofxuus. (Altered from Metschnikott"). 



1. A cysticercus-like form, with five (or more) areas of proliferation on the wall of the 

 bladder. 



2. Each of these becomes hollowed out to form a small sac, destined to drop into the central 

 cavity. Various stages of formation are shown (a, b, c). 



8. After becoming free in the central cavity, each sac gives rise to head, and so becomes a 

 cysticercoid as in previous cases. 



4. A Coetmrus, from the brain of the sheep; the numerous scolices, of various ages, -arise 

 by invaginations of the wall of the bladder. 



6. Echiiincoccu* ; the secondary bladders arise at a, 1>, c, in the same way as the scolices 

 in Fig. 2. Each then develops numerous heads by eversion of the wall of the bladder, into 

 which they may be secondarily withdrawn. At m is shown the ideal mode of origin, so as to 

 indicate the relation of head to daughter bladder, which is thus comparable to the "fore body" 

 of cysticercus. 



6. A St)>hylocyfti8 (S. glomeridis) as an example of external budding by a cysticercoid. 

 Each daughter cysticercoid develops a single scolex, the aperture of invagination of which is 

 shown. 



In these latter cases Coenurus, Polycercus, and Echinococcus 

 there is a process of gemmation, so that from each egg, not one only, 

 but several tapeworms will arise. Another kind of asexual repro- 

 duction is met with in the case of a cysticercoid occurring in Glomeris, 

 where by successive branching, and by the external gemmation of 

 secondary cysticercoids, a complex organism, known as a " Staphy- 

 locystis," arises (comparable to the sporocyst of D. macrostomum), 

 (Fig. XXXIII. 6). 



In those metacestodes called " Urocystis," the tail of the original 

 cystozooid drops off and becomes transformed into a second cysto- 

 zooid, which proceeds to form a second scolex. 



But even now that the life-cycle of these tapeworms is understood, 

 and the relation of the "cystic" form, occurring in parenchymatous tissues, 

 to the " segmented worm " living in the alimentary canal, is perfectly satis- 

 factorily established, owing to the investigations of van Beneden, Kiichen- 

 meiater, Leuckart, and others, there still remains a matter which is even 

 yet one of controversy, viz. to what degree does this history illustrate 

 Steenstrup's Theory of the Alternation of Generations. Up till recent times 

 the general opinion on the matter, founded as it was on an acquaintance 



