78 PLATYHELMINTHES CESTODA CHAP. 



others; those of mammals, by a third set of characters. The 

 young stages of the Cestodes of Sharks and Eays occur encysted 

 in the body -cavity, or in the pyloric appendages, of Teleosteans, 

 which probably swallow them along with those invertebrate 

 animals upon which they prey. The larvae of the Cestodes of 

 carnivorous mammals or piscivorous birds, live respectively in 

 herbivores and fishes, but how the latter are infected we know 

 in very few instances. Cestode larvae are known to occur in many 

 Invertebrates, and occasionally are taken free swimming in the 

 sea, presumably crossing from one host to the next. Cteno- 

 phores, Siphonophores, Copepods, Ostracods, Decapods, various 

 Molluscs especially Cephalopods, Earthworms, and other Anne- 

 lids, are the intermediate hosts of these larvae (see Fig. 38), 

 the fate of which, however, has been determined in but few cases. 



Occurrence of Cestodes in Man. 1 Tape- worms, either in 

 the adult or larval stages (bladder -worms), have, from ancient 

 times, been known to occur in man, and in the animals that serve 

 him as food. Until comparatively recent times, however, the 

 true nature of these parasites, and particularly of " hydatids " 

 (cystic larvae), was unrecognised. Up to the seventeenth 

 century the larvae were regarded as abscesses or diseased growths 

 of the affected organs, and it was only at the close of that century 

 that their animal nature was even suggested. Even at the 

 beginning of the nineteenth century, three modes of origin of 

 Cestodes by " generatio aequivoca " from the tissues of the body, 

 or by the union of previously distinct proglottides, or again by 

 metamorphosis of free-living worms drunk with water by cattle 

 or birds (as Linnaeus suggested) were still variously held, at a 

 time when Malpighi, Pallas, and Goeze had recognised the true 

 connexion between the cystic and segmented states of Taenia 

 crassicollis (the cat tape-worm), and when Goeze had seen the 

 eggs of Taeniae, and Abildgaard 2 had even conducted the first 

 helminthological experiments (conversion of the larval Schisto- 

 cephalus, Fig. 40, into the adult form). 



Generally speaking, " a tape-worm " in Western Europe will 

 prove to be Taenia saginata Goeze (the beef tape-worm, Fig. 39, A), 



1 Leuckart, Die Parnsitcn d. Mcnschen [English trans, by W. E. Hoyle] ; 

 Blanchard, Traite dc Zoologic mtdicalc, 1893. 



2 For a full account of the history of this subject sec Leuckart, Parasitcn <!. 

 Mcnschen, p. 28 ; Braun, loc. cit. Bd. iv. p. 929 ct scq. ; Huxley, Collected Essays, 

 vol. viii. p. 229. 



