STUDIES IN ANIMAL LIFE. S3 



their efforts till a hole is made large enough for 

 them to slip in by elongating their bodies. Once 

 in, they dismiss their tails as useless appendages, 

 and begin what is called the process of encysting 

 that is, of rolling themselves up into a ball, and se- 

 creting a mucus from their surface which hardens 

 round them like a shell. Thus they remain snugly 

 ensconced in the body of the insect, which in time 

 develops into a fly, hovers over the pond, and is 

 swallowed by some bird. The fly is digested, and 

 the liberated Cercaria finds itself in comfortable 

 quarters, its shell is broken, and its progress to ma- 

 turity is rapid. 



Yon Siebold's description of another form of em- 

 igration he has observed in parasites will be read 

 with interest. "For a long time," he says, "the 

 origin of the thread- worm, known as Filaria insec- 

 torum, that lives in the cavity of the bodies of adult 

 and larval insects, could not be accounted for. Shut 

 up within the abdominal cavity of caterpillars, grass- 

 hoppers, beetles, and other insects, these parasites 

 were supposed to originate by spontaneous genera- 

 tion under the influence of wet weather or from de- 

 cayed food. Helminthologists (students of parasitic 

 worms) were obliged to content themselves with 

 this explanation, since they were unable to find a 

 better. Those who dissected these thread-worms, 

 and submitted them to a careful inspection, could 

 not deny the probability, since it was clear that they 

 contained no trace of sexual organs. But, on di- 

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