FLATWORMS 



and their reproductive organs are comparable with those of turbellarians. 

 The eggs, which consist of a single zygote and a number of yolk cells 

 surrounded by a shell, are laid free in the water or loosely attached to the 

 host. A ciliated larva hatches from the egg, almost immediately attaches 

 to the host, and transforms into a miniature adult. The life cycle is simple 

 as compared with that of digenetic trematodes. 



Digenea. The digenetic trematodes are internal parasites having at least 

 two species of hosts in the course of the life cycle. In their adult stages, 

 Digenea occur almost exclusively in vertebrate animals, usually infesting the 

 blood, the urinary bladder, or the digestive tract and its appendages, the 

 lungs and liver. Each species has its particular habitat within the host, and 



Mouth - 

 Pharynx. 



Ventral sucker 



Excretory tubule 



^ — Vas deferens 



Vas efferens. 



Odtype 

 Ovary 



Seminal receptacle 



Testis 



Excretory 

 bladder 



Fig. 11.14. Clonorchis sinensis, the Chinese liver fluke. A, photomicrograph of a stained 

 and mounted specimen. B, diagram of a similar specimen to show the general structure. Note 

 particularly the two large, branched testes, from which the generic name is derived, and the 

 fact that the coiled uterus is filled with shelled eggs. (A, photograph courtesv Ward's Natural 

 Science Establishment, Inc.; B, adapted from D. L. Belding, Textbook of Clinical Parasitology, 

 second edition, copyright 1952 by Appleton-Century-Crofts, Inc., printed by permission.) 



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