94 THE CESTOIDEA 



definite organs of sense, are likewise absent in the adult. The 

 epidermis, which has sunk into the parenchyma, secretes a thick 

 cuticle, as in the Trematoda. In the parenchyma certain lime- 

 secreting cells are developed in greater or less number. Organs of 

 fixation are developed in a characteristic but varied form at one 

 extremity of the worm. 



The egg gives rise to a six-hooked embryo or " onchosphere," 

 which gains an intermediate host ; from it some form of " bladder- 

 worm " is usually developed, which has to reach a vertebrate as a 

 final host, in order to attain maturity. 



Historical Account. There can be little doubt but that tapeworms 

 have been known to mankind from very early times, for those 

 infesting domestic animals are sufficiently large to catch the least 

 observant eye; and even such "bladder -worms" as Cysticercus 

 cellulosae, C. tenuicollis, and Echinococcus must have been met with, 

 and recognised as foreign bodies, in the carcases of animals slain 

 for food or sacrifices. Moses probably was acquainted with them, 

 when the pig, rabbit, and hare were forbidden to the Jews. The 

 Greeks gave the expressive name ^aXafat ( = hailstones) to these 

 " hydatids," and some authors refer to the method, still employed, of 

 examining the tongue of the living pig in order to ascertain their 

 presence. The tapeworms were termed cA/xti/fles TrXarctai : and 

 Aristotle was aware that they were attached to the wall of the 

 intestine, whereas the nematodes or a-rpoyyvXai were free therein. 

 At an early period (1592) at least two different cestodes were 

 distinguished as inhabiting man (Taenia and Bothriocephalus\ and in 

 the latter half of the seventeenth century the tapeworms and bladder- 

 worms of domestic animals, and later of wild animals killed for food, 

 etc., began to receive attention. But for a long time, even after a 

 considerable number of naturalists had been working on the subject, 

 the relation of these two stages was unknown, or only vaguely 

 guessed at, till Pallas and Goeze recognised that the head 

 contained in the bladder- worm is capable of evagination on 

 compression, and resembles the head of certain tapeworms. The 

 earlier systematists (Zeder, Rudolphi) separated the bladder- 

 worms, or " Cystica," from the tapeworms, or " Cestoidea," as dis- 

 tinct orders ; but Blainville and Dujardin united the two groups, 

 as being related to one another. P. J. van Beneden rightly 

 regarded the " Cystica " as some normally occurring stage in the 

 life-history of the " Cestoidea." Von Siebold, on the other hand, had 

 put forward the theory that the bladder- worm is some stage in the 

 history of a tapeworm which has gone astray in the wrong animal, 

 and, undergoing hydropic degeneration, is destined to die, unless this 

 animal is eaten by the proper host, when the bladder- worm will not 

 only live in the intestine of the host, but will give rise to a tape- 

 worm. This idea of von Siebold's received considerable support 



