TAPE- WORMS. 



687 



system, consisting of fibres that run lengthwise throughout the body, and 

 of others that pass from wall to wall of the segments. It also possesses a 

 nervous and excretory system, the former repre- 

 sented by a ganglion in the head, whence a pair 

 of nerves run backwards along each side of the 

 segments to the end of the body. The excretory 

 organs consist of a circular canal lodged in the 

 head, from which spring four branches, one cor- 

 responding to each sucker ; two of these traverse 

 the sides of the body, and, becoming united in 

 the last segment, open by a common aperture on 

 its hinder edge. They are also put into com- 

 munication with each other in each segment by 

 means of a transverse canal, which runs along its 

 posterior border. Of mouth and alimentary canal 

 there is no vestige, the animal gaining its nutri- 

 ment by absorbing the fluids in which it floata. 



A Tape- worm is unable to propagate its kind 

 within the host in which it is lodged. But 

 each segment contains a complete set of organs 

 necessary for the purpose, and when it is ripe 

 and breaks off in the manner mentioned above, 

 it is found to be charged with eggs. Such a 

 segment, called a proghttis, makes its way to the 

 exterior, and, bursting, sets free the eggs, which 

 are thus disseminated. Sooner or later some of 

 them, in connection with food or water, are 

 swallowed by a pig. Each egg is then hatched and 

 gives rise to the young known as the proscolex, 

 a minute organism provided with three pairs of hooks (Fig. 12, e). By 

 means of its armature, the proscolex bores it way through the coats of 

 the pig's alimentary canal and enters its blood-vessels. It is then 

 carried by the blood stream to some such organ as the brain, lung, 

 muscles, etc., where it ultimately becomes lodged. Then growth sets in in 

 earnest, the proscolex loses its hooks, and, enlarging, becomes converted into 

 a bladder. Pork thus infested is technically called meaaly (Fig. 12, /). 

 At one point of the bladder a deep depression arises, and at the bottom of 

 this the hooks and suckers of the tape-worm head are developed; the 

 depression is then pushed outwards until it becomes a process, carrying at 

 the tip the head which is thus formed outside in. But beyond this stage, 

 known as the Gysticercns or Bladder-n'orm, the parasite cannot pass without 

 again changing its host. This is effected when the meat of an infested pig 

 is eaten in an uncooked state by a human being, whereby the Cysticercus 

 is introduced into the alimentary canal, the bladder is digested, and the head 

 or Scolex, attaching itself to the wall of the intestine, starts growing m 

 length, until it assumes the adult form and breaks up into proglottides. 



Another tape-worm, common in man, and presenting the same cycle of 

 development as Tcenia solium, with the exception that oxen are the inter- 

 mediate hosts, is Tcenia saginata, which is larger than the one just described, 

 and may be at once recognised from it by the absence of the circlet of hooks 

 on the head (Fig. 12, a). 



A third species that infests mankind, but is of more frequent occurrence 



Fitf. 12.— TiPE-WORMS. 



Head of Tcenia saginata. 

 Hoad of Toiiiia solium. 

 Head of J3uthriocephalus 



laius. 

 Ripe Segments of Tcenia 



solium 

 Hooked Embryo of Tcenia 



solium. 

 Measly Pork infested with 



Cysticercus. 



