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45 



Chapter IV. 



THE TAPEWORM. 



The tapeworm is a mouthless organism living when adult in 

 the alimentary canal of vertebrates. It consists of a chain of 

 segments. One end of the chain adheres to the lining of the 

 intestine by means of suckers and hooks, whilst the other end 

 lies freely in the cavity of the gut. The attached end is called 

 the ' scolex,' and this scolex buds off the segments, which 

 increase in size and complexity as they are pushed further and 

 further away from their point of origin by the interpolation 

 of new segments. Finally, the larger segments are detached 

 singly and are discharged with the faeces of the host. 



Two common species inhabit the intestine of the dog. 

 The discharged segments of the one species, Dipylidium 

 caninum, adhere to the hairs around the dog's anus. A dog- 

 flea larva feeding on one of these segments introduces into itself 

 one or more eggs which develop into tailed larvae, or cysti- 

 cercoids. Should a dog when snapping at the fleas swallow 

 one containing a cysticercoid, the latter attaches itself to the 

 dog's intestine, becomes the scolex, and proceeds to bud off 

 segments, so producing the mature tapeworm or ' Strobila ' 

 (pine-cone). The eggs from the mature segments of the 

 second species, Taenia serrata, are discharged on to the grass, 

 and if eaten by a rabbit develop into ' bladder-worms ' (see 

 p. 345) attached to the mesenteries or inhabiting the liver. 

 On being eaten by a dog, each bladder- worm excysts, attaches 

 itself to the dog's intestine and buds off segments as in the 

 first species. Thus the life-history of a tapeworm consists 

 of an egg, a larva, a cystic stage, and a segmented adult stage, 

 passed under different conditions and in different hosts. 



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