214 METAMORPHOSES OF MAN 



and gives rise to a scolex (ciliated larva), which then 

 produces its strobila (sporocyst). Prom the latter 

 spring several proglottides (cercarise), which at first 

 swim for some time around the animal by which they 

 have been hatched. When the period of their meta- 

 morphoses arrives, those which attach themselves to 

 stones, leaves, and such-like, soon perish ; but some 

 of them invariably discover the larva of an insect or 

 mollusk adapted to their wants, and pierce its integu- 

 ment. There they remain, till their temporary host is 

 in his turn devoured by some frog or water-bird, and 

 it is in the latter alone that the young Distomum 

 completes its organization, and acquires its definitive 

 characters. 



These strange migrations, which are accomplished 

 by a process which seems very well calculated to 

 destroy the vitality of the Helminthes, occur also 

 among the cestoid and cystic worms. For there it 

 has been proved by direct experiment ; and the result 

 of these experiments has been to show that these two 

 groups, which hitherto had been almost universally 

 regarded as distinct, are really but a single one. The 

 quasi-cystic worms are only a developmental phase of 

 the cestoid Helminthes. The credit of having formed 

 this conclusion, from a series of observations and 

 perseveringly followed researches, is due to M. Van 

 Beneden ; that of having demonstrated it by precise 

 experiments belongs to M. Kuchenmeister.* Thanks 

 to the labours of these two naturalists, and to those 



* "Kapport sur le grand Prix des Sciences physiques pour 

 1853," by A. De Quatrefages ; " Comptes Kendus del'Academie des 

 Sciences," 1853; and "Annales des Sciences naturelles," quatrieme 

 sene, t. l er . 



