THE WORMS 



51 



first and scarcely showing the segments which finally be- 

 come so prominent a feature. 



When carefully examined, a two-lobed brain is found 

 in the " head," and from it nerves extend the entire length 

 of the body, followed throughout their 

 course by the tubes of the excretory 

 system ; also each segment contains a 

 perfect reproductive system, so that 

 even if it be separated from the others 

 it may continue to exist for a consid- 

 erable length of time. Furthermore, 

 the tapeworms are surrounded by the 

 predigested fluids of their host ; a 

 special alimentary canal is therefore 

 superfluous, and all traces of it have 

 disappeared. 



50. Development. As the animal 

 clings in this passive way to the body 

 of its host the segments, loaded with 

 eggs ready for development, separate 

 one after another from the free end 

 of the body, pass to the exterior, and 



slowly crawling about like independent organisms, lay great 

 numbers of eggs, which may find an intermediate host as in 

 the life cycle of the liver fluke, and so in time find their 

 permanent resting-place. Fortunately in all these parasitic 

 forms, though an inconceivably great number of eggs are 

 laid, only a comparatively few reach maturity. Even these, 

 however, may cause at times great destruction among the 

 higher, and especially our domestic, animals, often doing 

 damage amounting to many millions of dollars per year. 



51. The tapeworm in relation to regeneration. It has 

 been known for more than one hundred and fifty years that 

 some of the lower animals possess to a surprising degree 

 the ability to regenerate parts of the body lost through 

 injury. The Hydra, hydroids, and some of the jelly-fishes 



PIG. 31. Tapeworm (Tcenia 

 solium). In upper left- 

 hand corner of figure is 

 the much enlarged head. 

 After LEUCKART. 



