III. CESTODA. 285 



and only come to their definitive position on the outside of the 

 scolex when the latter is protruded as one would turn out the 

 finger of a glove. The further development follows when the 

 cysticercus is taken into the stomach of the new host. When 

 man, for instance, eats infected ('measly') pork, the cysticerci are 

 freed by action of the digestive juices and later the scolex is 

 everted. The embryo passes to the intestine, becomes attached 

 and, surrounded by nourishment, begins to grow, the bladder 

 remaining attached to the hinder end, and soon the formation of 

 proglottids begins in the middle piece connecting the bladder with 

 the scolex. So rapid is the growth that in ten or twelve weeks 

 Tcenia solium begins to set proglottids free. 



In cases where the bladder reaches a considerable size it has the power 

 of producing more than a single scolex. The bladder of Coennms cerebralis, 

 which lives in the brain of sheep, produces hundreds of scolices. The num- 

 ber is even greater in Tcenia echinococcus, in which the bladder increases 

 by budding for some time, and by the formation of numerous daughter 

 bladders produces marked tumors in the liver of man and domestic animals, 

 before the formation of scolices begins. In the interior of each daughter 

 vesicle appear a number of brood vesicles, each of which produces numbers 

 of scolices, so that from a single six-hooked embryo thousands of scolices 

 can arise (fig. 253). This extreme case stands in contrast to others which 

 connect with the development of Bothriocephalus, in which the cysticercus 

 is replaced by a cysticercoid (fig. 248). Here there is no infiltration and 

 the scolex is closely enclosed by an envelope comparable to the bladder 

 wall. 



All of this is of importance in the correct conception of the development 

 of a tapeworm, which was earlier believed to be a complicated alternation 

 of generations; the bladder to be a stage which by endogenous budding 

 produced scolices; the scolex, in turn, a stage which by terminal budding 

 produced the sexual animals, the proglottids, and the tapeworm itself a 

 chain of individuals, a strobila. This view, so easy to learn, so easily ex- 

 plaining the development, contains two errors. The bladder is not an inde- 

 pendent generation, but only the precocious hinder end of the scolex. The 

 tapeworm is not a colony, but a single animal; the proglottids are not in- 

 dividuals, but specialized parts of a single whole. This view is confirmed 

 by a comparison with other forms. The Caryophylla3ida3 (fig. 249) are 

 single bodies, the anterior end elongate and taking the place of the scolex, 

 while the broader hinder part contains a single hermaphroditic apparatus. 

 In the Ligulida3 the body is still unjoiuted, but has increased in length and 

 contains numerous sets of sexual organs. This duplication of the repro- 

 ductive apparatus explains the appearance of proglottids. 



Family 1. CARYOPHYLL^ID^: (Cestodaria). Cestodes without ace- 

 tabula, simple sexual apparatus, scolex and proglottis not differentiated. 

 Distinguished from tremat9des by absence of digestive tract. Larval stages 

 in invertebrates, adults nearly always in fishes. Caryophyllceus (fig. 249) 



