126 ANIMAL BIOLOGY 



in the intestine, or liver of its host. Of course the chances 

 are very small that any one embryo will make all the con- 

 nections necessary for a successful life history. This 

 circumstance is offset in part by the enormous number of 

 eggs produced in the beginning, and in part by the fact 

 that each embryo may produce many others provided it 

 makes the first necessary connection with the body of 

 a snail. Truly this seems to be a very roundabout and 

 wasteful method of perpetuating the species, and calls 

 to mind what the poet Tennyson said of Nature: 



"So careful of the type she seems, 

 So careless of the single life." 



No matter how many eggs or larvae fall by the wayside 

 so long as the race of liver flukes makes the journey from 

 sheep back to sheep again. Each species of animal solves 

 the problem of getting through the world in its own way; 

 and the ways that are followed are often very devious and 

 peculiar. 



The Cestodes, or tape-worms, are also parasitic, and they 

 have been addicted to such a life for so long that they have 

 lost all traces of an alimentary canal. Instead of digesting 

 their own food they live by absorbing the digested food 

 in the alimentary canal of their host. With one possible 

 exception, the Cestodes are parasitic in the adult state in 

 the intestine of the vertebrate animals. 



Most Cestodes are divided into a number of segments, 

 or proglottids. In the larger human tape-worms which 

 may reach a length of thirty feet there may be over 1000 

 of these segments. One end of the body is usually fur- 

 nished with suckers and sometimes also with hooks for 

 attachment to the wall of the intestine. Behind the at- 

 tached end new proglottids are continually formed and 

 they gradually increase in size and become more mature 



