GENERAL ZOOLOGY 



there is also a high degree of host-parasite specificity, as a result of which 

 each stage of a particular parasite is limited to one or a few related species 

 of host. The gut in the Digenea is typicalK- composed of two main branches 

 which separate from each other just behind the anteriorly placed pharynx and 

 end blindlv near the posterior end of the body. The nervous system is best 

 developed in connection with the organs of attachment. The animals are 

 monoecious, with few exceptions, and their reproductive organs are compar- 

 able, with modifications, to those of turbellarians and monogenetic trematodes. 



A characteristic feature of the group is the life cycle, which is one of the 

 most complex in the Animal Kingdom. As an example, we shall describe the 

 life cvcle of one of the Oriental liver flukes of humans, Clonorchis sinensis 

 ("branched testes, living in China") (Fig. 11.14). The adult of this species 

 is found in the bile passages of man. It produces large numbers of eggs, each 

 of which consists of a spheroidal shell containing a single zygote and 

 numerous yolk cells, like the eggs of monogenetic trematodes. These eggs pass 

 through the bile duct into the intestine and are voided by the host in its 

 feces. Although embryonic development is completed within the shell, the 

 resulting larval form, the miracidium (Fig. 11.15), does not emerge until the 

 egg has been ingested by a particular kind of snail. A small number of 

 species belonging to a single familv of snails are known to serve as inter- 

 mediate hosts. Within the body of the snail the ciliated miracidium hatches 

 and immediately makes its way into the tissues, where it transforms into a 

 second larval form, the sporocyst. This is a thin-walled, sac-like organism 

 which has few well-developed structural features but contains a mass of 

 germinal tissue, from which are produced members of the next larval genera- 

 tion, each of which is known as a redio (named for the noted seventeenth- 

 century Italian microscopist, Francesco Redi). The rediae migrate into the 

 blood sinuses of the snail's liver, where each produces internally a group 

 of "germ balls," each of which develops into a tailed larval stage, the 

 cercaria. Several cercariae at once burst through the wall of the redia, 

 make their way to the outside, and take up a free-swimming existence. When 

 a cercaria encounters any one of several dilTerent kinds of fresh-water fishes, 

 it bores through the skin, sheds its tail, and becomes encysted in the flesh of 

 this host. This stage, consisting of the body of the cercaria which has 

 secreted a cyst about itself and developed somewhat toward adulthood, is 

 known as the metacercaria. Cysts containing viable metacercariae may be 

 ingested by humans eating insufficientlv cooked fish; in the duodenum of the 

 final host the metacercaria emerges from its cyst and makes its way into the 

 liver, where it soon becomes a fully matured adult and begins to produce eggs. 



This life cycle, with its interesting emphasis on reproductive efficiency, 

 may be summarized as follows: one monoecious adult, in which self-fertilization 

 appears to be the rule, produces thousands of eggs daily over a period of 

 years; each egg develops into one miracidium; each miracidium becomes one 

 sporocyst; each sporocyst produces many rediae; each redia gives rise to many 

 cercariae; each cercaria transforms into one metacercaria, which becomes one 



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