AND THE LOWER ANIMALS. 213 



the ciliated larva of the distomum is the scolex, and 

 its sporocyst is the strobila. Bach Cercaria is a pro- 

 glottis ; but in this instance the proglottis, before 

 reaching the state of perfect distomum, undergoes a 

 series of true metamorphoses, quite like those of 

 insects in general, and of the Diptera especially. 



These complex phenomena are further complicated 

 by a very curious circumstance. 



We met with insects living at first, when in the 

 larval state, in water, and then in air, when the 

 successive metamorphoses carried them to the con- 

 dition of perfect insects. According to the periods 

 of life which they have reached, these insects then 

 inhabit different media, distinct worlds. 



Now, Trematodes present a similar state of things, 

 only in their case the media, or worlds, through 

 which they pass in order to possess the conditions 

 necessary to the progress of their development, are 

 so many distinct species of animals. They must travel 

 from one to another, and these migrations are most 

 frequently accomplished by a process as simple as it 

 is unexpected. The parasite is subject to the same 

 fortune as the individual that carries it. When the 

 latter is devoured by some other animal, the Hel- 

 minth is also eaten, and travels along with the 

 aliment, of which, indeed, it is a part. According as 

 its new habitation is or is not suited to its require- 

 ments, it dies and is digested, or resists the solvent 

 action of the surrounding liquids, and begins a new 

 phase of its development. 



For example, a Distomunr's egg falls upon the leaf of 

 some aquatic plant and is swallowed by a Lyrangea or 

 Paludina ; it is hatched in the interior of the mollusk, 



