194 ANIMAL PARASITES AND MESSMATES. 



free and attached to the lower surface of some medusse. 

 It exists occasionally in considerable numbers on the 

 internal surface of some Acalephae of the ocean and of 

 the Mediterranean. Claparede has also observed another 

 free cercaria which bears the name of Pachycerca. 



Some of the cercariee are very tenacious of life; we 

 have kept some alive in fresh water during a whole week 

 in the month of November, and on the last day they 

 were still active (Cercaria armata). We sometimes find 

 the cercarian age passed over, and the young distomes 

 appear abundantly without tails in the sporocyst. We 

 have seen an example of this in the Buccinum undatum 

 of our coasts. This latter generation assumes in every 

 case a very different form from that which preceded it. 



Lodged and nourished without expense in the succu- 

 lent parenchyma of their victim, the cercariae grow 

 rapidly, and as soon as their caudal oar is developed; 

 they tear asunder the membrane which encloses them, 

 and abandon their host in order to live freely as tad- 

 poles. Some fine day, tired of their nomadic life, they 

 choose another host, get rid of their tail, fold themselves 

 up in a winding-sheet, like a chrysalis about to become 

 a butterfly, and concealed in a sac, which is designated 

 by the name of cyst, they wait patiently for days, 

 weeks, or years till their host is swallowed by the 

 creature intended to lodge them. The cyst is set free in 

 the stomach of the latter host, its envelopes are dis- 

 solved in the juice secreted by its enclosing membrane, 

 and with its whole establishment the worm recovers its 

 liberty in this new abode. 



The encysted cercarise pass thus with arms and bag- 

 gage into the stomach of a new host. Their envelopes, 



