314 



PLATYELMIA ; TREMATODA ; CESTOIDA 



ing to which are found in the parenchymatous organs of animals, 

 although some infest their intestines. In the development of 

 some of the Trematode worms, we meet with an alternation of 

 generations, as it is called, of a very singular nature. The yelk 

 gives origin to a flat, ciliated embryo, furnished with a pair of 

 eye-like points at the anterior extremity. In the interior of this 

 a sac-like body capable of motion is produced (by gemmation) ; 

 and this, getting access to the body of some animal different from 

 the one in which its parent was parasitic, gives origin by another 

 process of gemmation to a multitude of little granules, which be- 

 come developed into small tailed animalcules, identical with those 

 which were formerly described as Infusoria under the name of 

 Cercarice. These break out of their sac-like parent (or nurse as 

 it is called), and swim about freely in the water, until they re- 

 commence a parasitic existence by boring their way into the 

 aquatic Iarva3 of Insects. Here they lose their tails, and enclose 

 themselves in a little cyst, to wait until the insect inclosing them 

 is devoured by some predaceous animal (fish, or bird), in the 

 body of which they are able to continue their development. The 

 most singular species of this group is the Diplozoon paradoxum, 

 a creature which consists of two exactly similar halves, united 

 by a narrow band in the middle of the body, as though two 

 separate animals had grown together at this point. This curious 

 animal is parasitic on the gills of the Sea Bream. 



925. In the CESTOIDEA, forming the third Order of Flat- 



no. 635. TJENIA SOLIUM. 



worms, the process of development is no less curious, although, 

 in many cases, the number of transmigrations appears to be rather 



